Experience! Gold! Thieves! - Kinder Surprise Houserules/Mods.

Your kind of party, eh?
Russ Nicholson for Citadel of Chaos.

Quicker Advancement.

• If you're playing with only three classes (OD&D style), shift down the XP tables so MUs advance as Fighters, Fighters as Clerics and Clerics as Thieves (or unchanged), or:
• Use 3e/4e/5e XP tables, and subtract that amount from corresponding older school class XP requirements. So, a BECMI Cleric needs 500 (3e) or 1200 (5e), a Fighter needs 1000 (3e) or 1700 (5e) to hit 2nd level. 

(This is one way of reducing the economy-busting amounts of gold that are an issue for some, and a way of speeding up advancement for the time-poor while keeping the staggered class advancement model)

Bonus Extra: if the table agrees, every time you roll a save, you get XP equal to the amount you missed or made the save by x10. Roll exactly your save value for 100 XP.

It's more book-keeping, but rewards general adventuring behaviour/risk-taking as you get something whether you fail or succeed. As your characters rise in level, it will make much less impact and you can drop it once it gets too cheese-paring.

Less Cash in Circulation.

Related to the lower XP requirement idea above and the fairly common trope that adventurers/heroes start off each story broke, hoping to make one big score and then end up cheated and broke again.

Characters do not keep ANY treasure that goes towards XP. 

It is out of your hands by the time the next adventure comes around, through debt, carousing, alms-giving, theft, gifts, living expenses (at the appropriate level of extravagance), cheats, gambling etc. You can even call it training costs. Narrate or hand wave this as your table likes.

Any cash (or choice items) you want to keep (in hand, in the bank, buried under a particular tree, put towards buying a castle, whatever) or spend on adventuring assistance/supplies does not count towards XP.

For extra misery, allow included living expenses to exceed the amount of XP you gain, so you end up in debt.

Are healing and curse removal costs inclusive or extras? 

More Thief Mods.

Because we just can't help it! 

Under either of the following (and I don't think they can work together), keep Hear Noise as a d6 skill (if that's in the system) and Backstab is unchanged.

Skills as saves: allocate your Thief skills to your saving throws (combine as appropriate), and test them on a d20 (or convert to %). 

You can swap a pair of saves/skills every time you gain a level.

You can apply Ability Score adjustments as agreed/appropriate.

Skills as hit rolls: each skill is an attack roll vs. unarmoured AC (or convert to %).

Your skill advances as a Fighter's attack progression, and leather/chain/plate can be used to represent grades of abnormal difficulty or increasingly stressful conditions.

In a mod of 2e AD&D weapon specialisation, the Thief can spend max. one of their NWP slots to specialise in a single skill at 1st level and get a +1 bonus. They can use a single additional slot gained at higher levels to specialise in another skill, or take their bonus to max. +2. This might also work with the 'skills as saves' method too.

Bonus Extras:

• Thieves open doors, locks, chests, disarm traps etc on an Open Doors roll, but use INT and/or DEX (depending on characterisation, and Dragonwarriors would suggest an average) to generate a STR bonus equivalent.
• Keep percentile skills, but you get to add your Prime Requisite % bonus to the base value.
• Thief skills % start at equivalent of 3rd level and scale from there.

Commentary.

Quick crude mods for a basic D&D adjacent game. Not playtested but someone's probably already tried all/most/some of these already in the last 40+ years.

Realise I could have held them back to pay this Joesky Tax I've heard about, but they've been hanging around my drafts and notes for a while now so I'm putting them out rather than letting them fester.

XP for saves is a rip from T&T.

Old School Thief Variant - Adventurer, Ranger, Rogue.

There comes a time in all our lives when we have to 'fix' the Old School Thief.

Considering they're part of the core text of D&D, they needed a lot of modding.

Description: Thief, adventurer, ranger, rogue, ruffian, swashbuckler, scoundrel, hero.

Whatever they call you, your traditional ancestors are still Cugel and the Grey Mouser. But roll high STR/ CON and you're also Conan or Fafhrd without having to multiclass or otherwise bend/break the rules.

You might want this kind of Thief to specify if they're more inclined to the rural, the urban or the dungeon environment, and tweak their abilities accordingly.

XP, Saving Throws, & Attacks: All as Thief.

Hit Dice: Depending on the base system, you start with 2d4 or 2d6 hit points, but gain only 1d4 or 1d6 per level as normal thereafter.

Prime Requisite: Dexterity.

Armour: Any, but as Thief only when you want to do Thief stuff.

Weapons: Any, but you prefer the lighter/smaller hand weapons for special ability reasons.

Alignment Restrictions: None, unless that's important for in your setting/system.

Special Abilities.

Thief Skills: Everything a Thief can normally do, but agree which of either Hear Noise probabilities or boosted percentiles you use before play begins.

If you have a CHA, INT or WIS bonus (highest applies), you can start casting from scrolls that many levels earlier but chances of failure do not change until your levels catch up.

Snipe: You can Backstab using a missile weapon if this is not already allowed.

Dual Wield: When fighting with two weapons, you get +1 to Hit or AC. This also applies to unarmed strikes. You cannot Dual Wield while performing a Backstab.

Choose which bonus to use at the beginning of each combat round, before you roll any dice. 

If you hit, use the higher rolled damage or choose which weapon inflicted the wound (for magic and poisoned weapons, for example)

If you have 18 STR, you can wield a max. d8 and d6 weapon simultaneously; STR 15, 16 or 17, a max. d8 and d4 or d6 and d6; all other STR scores, the max. is d6 and d4.

(This presumes you don't already have Dual Wielding going on anyway)

Cleave: If Fighters get extra attacks vs. <1HD opponents, so do you but only when Dual Wielding.

Surprise: If you are alone or with no more than three other Thieves in leather or no armour, your first surprise attacks count as Backstabs.