So, let's say you're doing a 2e clone. It's the last TSR system remaining to be cloned, so why not? I mean, you could start with TSR 1070, the Second Edition's version of the Red Box. But I'm getting ahead of myself. Back to Rogues/Thieves/Sneaky so-and-so's that get to do cool stuff.
Myth and Magic by New Haven Games has a mechanic for proficiencies that is addressed by a d20 roll. Basically, Roll over this number (24 - STAT +2) with a d20. For proficiencies that it all well and good. Yet there is only one option to improve with this system. Buy the proficiency again and you get (24 - STAT +4) as the target number. Tweaking this formula a bit, let's restate the Rogue's ability table.
Pick Pockets - 6
Open Locks - 5
Find/Remove Traps - 4
Move Silently - 5
Hide in Shadows - 4
Detect Noise - 6
Climb Walls - 15
Read Languages - 0
At First level, the Rogue gets twelve points to spread out across these skills as they see fit. It is recommended that no more than six are given to any one skill, but that will be up to the GM. No skill can be raised higher than 20. There will be a table to show how high DEX affects each skill. A minimum DEX of 16 is required to have any affect on skills. High WIS can provide bonuses to Read Languages. (I know, not INT.) Just use the Attribute Bonus, no table needed.
DEX 16 | +1 Open Locks
DEX 17 | +1 Pick Pockets, +2 Open Locks, +1 Move Silently, +1 Hide in Shadows
DEX 18 | +2 Pick Pockets, +3 Open Locks, +1 Find/Remove Traps, +2 Move Silently, +2 Hide in Shadows
Once the rogue sets up their focus of abilities, future success is determined by looking at their score and figuring that Rolling over 24 - SCORE will determine success.
Okay, that's not enough for you? How about one mechanic to rule them all. Here's COMBAT:
Roll Over on a d20: 24 - AC + Weapon Proficiency + Modifiers. Weapon Proficiencies are purchased in +2 increments, Warriors start with 4 Weapon proficiencies, purchases can be stacked. My 1st Level Warrior is proficient with the longsword X2, longbow, and short sword. He has a STR of 18/80 giving him +2 to hit. He is using a +1 longsword to attack a creature with an AC 5. All of this boils down to 24 - 5(AC) + 4(Proficiency) +2 (STR) + 1 (Sword) = 12
Roll 12 or better to hit. Why does a 1st level Warrior have a +1 Longsword? Relax, it's an example. I wanted to provide a good example. In a real campaign, odds are VERY low that a 1st level Warrior would have a +1 weapon. If you do the math, a 1st level Warrior with a double weapon proficiency will calculate to a THAC0 of 20, THAC1 of 19, etc. Mages, with only one weapon proficiency, still calculate to a THAC0 of 20, THAC1 of 20, THAC2 of 20, THAC3 of 19 at 1st level.
So does that work for a simplified (as much as Second Edition was simple) mechanic to convert 2e stuff?