Kinder Surprise/Mix-and-Match Magic System(s).

I always wanted a magic system that was stuffed to the gills, but managed to be defined and unified. 

But essentially it boils down to I didn't see why a 1st level BECMI Magic User shouldn't also have a chance to cast summon byakhee.*

This isn't that system, but it is a rambling collection of resources, ideas and suggestions that might be of use. It's sort of system agnostic, scaled to 3-18 Ability Scores and d20 or % mechanics, and I've successfully used 3d6/4d6D1 homebrew mechanics with some of it. 

Source material includes Fighting Fantasy, Call of Cthulhu (2e/5e), Dragonwarriors, iterations of D&D, Tunnels & Trolls 5e, Dragonroar, a little bit of Runequest. 

Here's a summary of a great magic system hack for AD&D by Black Dragon Games/ Dweller of the Forbidden City (Twitter / blog) - it's a better general synopsis of some of the things I want than I can easily articulate:

• Magic Users can cast a number of spells per day equal to their Level, whatever the level of the spell.
• To learn a spell, they must successfully roll their % Chance to Know (Intelligence Table II: Ability for Magic Users, 1st ed. PHB p. 10) or they can never learn it.
• They can learn/know a maximum of (Intelligence + Level) spells.
• To cast a spell successfully, % roll vs. Chance to Know (as above) + 1% per Level (max. 99%).
• The % chance of suffering harm from a failed casting is (Spell Level x5) - caster Level.

The original Twitter thread should be here.

You could boil it down to a d20 Intelligence check adjusted by (Spell Level - Magic User Level), used for learning, casting and possible harm, with natural 20 always failure/harm and natural 1 always a success. 

This is slightly less punitive than the system in 2e Masque of the Red Death, but that has Ravenloft Powers Checks as consequences for casting spells - not just failure. As Powers Checks are %, you can bolt them straight on to your home system if you want them as an addition/alternative.

Another % hack for D&D spell casters is to use the following from 1st ed. DMG, p. 128 (it's originally for scrolls, but you get the idea):

This was also suggested by BDG/DofFC in a Twitter exchange touching on crossing the magic systems of older edition D&D and CoC (seriously: BDG has been a real educator in AD&D, a system I thought I knew better).

Because BRP (more specifically, older edition RQ and CoC) uses %, it is easy/easier to bring spells across into a D&D adjacent using these mechanics. Working out the levels of spells that do not have them could be as simple a matter as saying 'the magic point cost equals the spell level', but you might prefer to convert them more carefully (any spell causing non-permanent characteristic loss could be 3rd level minimum; permanent loss, 5th or 6th).

And, because we're still talking %, you could port the chances of spell failure while wearing (metal) armour for Sorcerers from Dragonwarriors (10/20/30% for ring/chain/plate), either as absolutes or as additional penalty. If you prefer, the progression could be leather/chain/plate, and answers why can't/doesn't a Magic User wear full plate (but has no problem using a shield, it would appear).

DW Mystics risk spell failure on a similar scale if wearing magical armour (10% chance per bonus point), which might also suit some Magic User types (or Clerics, if you play an arcane vs. divine setting). This might also apply to protective magical items, too.

Using Spells From Other Systems.

There's enough points of similarity between D&D, CoC and DW, that you could use spells directly from one system in another without too much change. 

Range, duration, area and general effect are no problem. There should be ample crossover material in systems for fire, acid, poison and so on. The 5% increments on a d20 work in both conversion directions. And subsystems peculiar to an individual spell will sometimes work on their own, outside of their origin system.

As mentioned above, CoC and RQ spells are D&D spell level equal to their magic point cost and DW spells cap at 10. For D&D, anything that would convert to a level greater than 9 can be treated as 9, but I'd go with 10 instead - representing magic that is rare, raw, complex, unfamiliar, alien; these spells are from the equivalent of a different universe, by some metric. Level equals magic point cost if you're going the other way.

CoC spells that cost/damage characteristics (temp or perm) can also do the same to D&D and DW stats, because they're within the 3-18 scale.

POW maps to Charisma, Wisdom and Intelligence in D&D (decide or randomise), and to Psychic Talent in DW (which could mean a character loses their spell-casting ability if they drop below 9 - whether a DW Assassin would lose any or all of their abilities is a question raised).

Spell damage can be taken directly from the spell descriptions, but bear in mind that these will have been calibrated for the game/setting and those expected hp totals (5-18 in CoC and never changing; min. 7 in DW and only +1 per level). D&D spells taken to CoC will be cast at the lowest spell or caster level possible; in DW, you can do the same, or scale them with the caster's rank.

DW flat damage is roughly the average of a D&D dice/die roll, and most DW weapons use the same die for Armour Bypass as they do damage in D&D. Older edition CoC/RQ weapon damage is not far off D&D.

Spells from 2d6 Fighting Fantasy SKILL/STAMINA/LUCK systems can be ported, using their STAMINA cost as magic point cost/ spell level. Bonuses/penalties from spells can be used directly. In the gamebooks, you usually wielded a sword and scored 2 damage per hit, so calculate damage as d6 or d8 for every 2 STAMINA damage (or part of/potential 2).

SKILL maps approximately to rank, Level/HD, to Hit, ATTACK/DEFENCE/EVASION, Dexterity and Intelligence; STAMINA to Strength and Constitution, maybe SIZ, too; LUCK to Charisma, rank, Level/HD, POW.

Some spells have analogues across systems, but should be treated as different spells (representing more or less developed versions, other magical traditions, lost spells, newly invented spells and so on). For instance, the DW Sorcerer spell invisibility is a 9th level spell (9th rank caster) but only 2nd in D&D.

Access to spells from non-D&D systems will give D&D Magic Users significant healing magic, so consider dropping or modding the Cleric class if you go whole hog for this kind of thing.

Intelligence Instead - Doing Away with Spell Levels.

A minimum Intelligence of 9 is necessary to be a Magic User in AD&D (and a Sorcerer in DW; Mysticism doesn't, because it's not 'academically demanding'), and you need a minimum 10 Intelligence to be able to cast a 5th level AD&D spell.

Thus, a spell could be given an Intelligence rating of x2 its level (with the option for 1st to 4th level spells to all have an Intelligence of 9), and this could be used to rate how difficult it is to learn/cast. Percentages would be 100 minus the equivalent chance to know, or based on the difference between spell and caster Intelligence, using the scroll table above.

Another option, using d20 Intelligence checks, would be to generate a bonus/penalty using the AD&D magic jar subsystem, or each 4 or part of 4 points of difference generates a +/- 1 mod to the roll, or some other match modifier method.

This obviously makes higher Intelligence Magic Users significantly better at learning and casting spells, but I tend to think of Intelligence as your character's Magic User-ness, rather than the intellectual and academic capacity of the character - player skill is, within some limits, character skill.

Use the caster level to calculate spell effect; negotiate or rule on whether the minimums in the spell descriptions apply under this regime.

If you're doing away with levels altogether, spell level for effect can be:

• 1st level
• minimum level of the spell cast
• caster level is equal to half INT (powerful)
• caster level is 1 + Ability Score bonus (as appropriate)
• 1 + how many STR you are willing to sacrifice (recovery rate to be determined; you can kill yourself doing this; other Ability Scores can substitute as appropriate)
• burn 1 hp to get the spell going, then hp equal to the casting level you want
• burn hp equal to the spell level, then +1 hp for every level you want to boost it by

Both hp burn and STR methods open up the possibility of others contributing, either by taking part in a ritual casting or as blood sacrifices.

Spells for Unlevelled Casters.

Whether you mean 0-level NPCs in a levelled game, or because you're not using character levels to measure experience, or because the caster is not a Magic User by class, they can cast daily spells equal to the relevant Ability Score bonus (or total, or average, of CHA/INT/WIS) - the amount will depend heavily on which edition of (say) D&D you use to calculate your bonuses.

In fact, you might want to extend these bonus spells to regular Magic Users, but I'd rule that these are front-loaded and not cumulative with those gained by level.

The Spellcasting Day.

If you follow spell memorisation RAW from AD&D (can't speak for the detail of later editions), you don't just wake up fresh and full of spells, or get up and leaf through your travelling spellbook over iron rations - you might not ever get to fill all your memorisation slots because you don't have enough time in the day. If you want to memorise a single 9th level spell, you need 12 hours rest/sleep and then 2 hours 15 minutes of meditation/study for that spell alone.

Dragonwarriors brings a more flavourful question of timing (dramatic and practical) to spell recovery. It's a magic points rather than spell slot system, and spell-casters know all spells of their level, but it works if you're not strictly Vancian. 

Sorcerers regain their spells/ magic points at midnight (precisely, apparently, so you can presumably set your clock by one). Mystics recover at dawn and Warlocks replenish their reserves at sunset - more nebulous and debatable. Are they connected to the sun, its light or absence? What about in the extreme north/south of Earth-like worlds?

DW Elementalists must perform rituals under specific conditions to regenerate their casting ability, opening a whole host of options/restrictions for casting traditions - sacrifices, lunar cycles, your horoscope, eclipses, astral conjunctions, lightning strikes, retreat to inner sanctums etc.

Duration.

Some alternatives to minutes/rounds/turns scaling to caster level:

• one battle: presupposes a combat spell; no chance of expiry mid-combat may take some of the drama out of things
• one encounter: not necessarily a combat spell, and can be very brief or an extended period, and depends very much on how you define an encounter
• as long as it needs/takes: utility spells, applied to the task at hand; can include spells that require a trigger to discharge
• concentration: at minimum, should be disrupted by casting another spell; injury is not necessarily going to spoil concentration (spell maintenance as serious discipline), but will likely be the first choice of opponents
• instantaneous: the bang-and-done spells

A spell duration of 'a day' should be more like the spell-casting day than a 24 hour countdown. Or measured as until you go to sleep (including KOs and sleep spells) or until you wake up - variant healing spells that demand a good night's sleep as part of the deal, or extraplanar communication, or go to sleep, wake up somewhere else.

Permanent spells might not be literally permanent, but practically so for game purposes: 7 days, a lunar month, 40 days and 40 nights, a season, a year-and-a-day, 666 days, 100 years.

Duration to when a condition is met or broken is a feature of folklore and myth, and works for all types of spells (except maybe the instantaneous ones, but even then there might be some room for exploration).

Spell Expiry Roll.

Lifted straight from Dragonwarriors: roll 2d6 at the beginning of each (6 second) combat round, on a 12 (or 2, if you prefer low rolls bad) the spell ends.

According to this source, a per minute Spell Expiry Roll would be 25% expiration chance each time.

(There's also this thread on predetermining DW spell durations, but I'm inclined to think that if you're going to do this, you don't need the mechanic in the first place)

For DW Warlocks, the Spell Expiry Roll is made on d20, the spell ending on a 20 (or a 1, if you prefer); with a 40% expiration chance minute-to-minute.

Or it could be an Intelligence check each round (or other unit of duration), or Strength (nod to Tunnels & Trolls).

Or cumulative rolls -on whatever dice you like- until the total exceeds your Intelligence, or your Strength, or the average/total of your spell-casting characteristics, or any of these plus your Magic User level.

Fatigue from Casting.

As much as I like the Psychic Fatigue mechanic from Dragonwarriors (roll equal or less than 13 + caster level - spell level or you can't cast another spell until the next dawn; 20 is always a fail), I haven't found a simple way to make it compatible with an/the envisaged unified magic system - one of its purposes is to mark a clear line between the magical technique/ tradition of Mystics and Sorcerers. [edit: I did, later]

However, actual physical fatigue related to spell-casting is baked into multiple systems (and much of fantasy/magical media via IRL beliefs, traditions and examples): Dragonroar spells use up Endurance, FF uses STAMINA, older edition T&T uses STR, going to 0 magic points in CoC knocks you unconscious.

And this bit (AD&D 1st ed. DMG, p.40), giving one of the reasons 'why Vancian?':

Using the BDG/DotFC system (for example), a level of fatigue could be the harm from a failed casting.

A simple fatigue progression for spell-casters I use (and for Light/Serious/Critical Wounds) is Tired/ Exhausted/ Spent, with non-cumulative penalties to all Ability Scores of -1, -3, then -6. Once Spent, you would immediately be placed in the out-of-action-but-not-dead-yet condition for your game if you took another level. And why not let the one after that be fatal?

Full or partial recovery from fatigue and full recovery of spell-casting capability are not necessarily linked.

Casting Beyond Your Limits.

I've not playtested this, because it's not come up, but the Magic Users I'm thinking of can cast their daily complement of spells (1 per level, or 1+level) without penalty - they can then attempt to cast spells beyond this, with the risk of failure and harm.

Either use BDG/DotFC % (if you're already using it, you could apply a penalty), or an INT check for casting and a CON (or STR) check (whether or not the casting succeeds) to see if you are drained and take a level of fatigue (see above).

Armour penalties (1 for every AC/armour point better than Unarmoured) would apply to these rolls - even if armour has no effect on normal casting - giving a mechanical reason for the legacy inclusion of unarmoured Magic Users (and something like a legacy Cleric, who wears heavy armour but chooses not to risk failure/fatigue once their daily spells are cast).

If you are trying to shoehorn Psychic Fatigue into a magic system that's already quite full (see above), a failed INT check/casting could indicate Psychic Fatigue - you cannot attempt to cast any more spells until your usual recovery condition/time, and you still take a CON/STR check for physical fatigue.

Option vs. Spamming.

If you try to cast a spell a second time in a spell-casting day, it is treated as if you were casting beyond your limits - you must roll to cast, and roll for fatigue; it also consumes a spell-slot.

Use this option to encourage variety and creative spell use/ to punish system mastery, depending on your point of view.

This option harks back to the little bit of Gygax earlier - you've used your Vancian privilege, and now you've got to pay a price. Imagine the spell as a brand on your spirit, repeat castings pressing it deeper into your sizzling ectoplasm.

A less punitive alternative would be to allow repeat casting (using spell slots as normal), but requiring a roll for Psychic Fatigue relating only to that specific spell - which is why it's worth knowing the D&D invisibility spell (for example) as well as the DW one.

IRL wild talents often claim to have faked phenomena (while still claiming the reality of previous events) because of the demand for reproducible, measurable effects. Whether you believe this or not, let's say that magic doesn't care about the scientific method. I think the same might be true in magically saturated settings, too.

What Was Left Out.

Lots of stuff: The Beholder Contracts (Great Wizards), Lankhmar (Black and White Wizards), Maelstrom (free-form magick), A Ghastly Affair (incantations, pacts, talismans, rituals etc.), A Mighty Fortress (the Scholarly Mage kit and bad weather with spell-casting), Grey Star the Wizard (magical disciplines, rather than spells), Runequest's division of magic into spirit, divine and sorcery.

Some of it might pop up later.

*I know LotFP effectively did this by making summon a 1st level spell, but as a kid in the 80s I couldn't be sure Gygax wouldn't come round and administer a punishment beating.