I'm just about finished with my Nano novel. I should be able to write the last ten pages or so tomorrow. So I'm taking a break tonight and finally getting around to the final post about the classic 4 classes and my house rules for them.
Back in the day, Thieves were always my favorite class to play. I think it had something to do with the promise of cool stuff that could be done with the Thief's special abilities, even if they didn't work all the time, and a good set of Elven Cloak and Boots, a Rope of Climbing, and a bag of holding made some of them redundant or useless.
Although my main character, Gwydion, whom I derived my internet gaming alias from, was a Fighter, my main Thief, Achaz the Younger (Achaz the Older got liquidized by a black dragon on his first adventure) was always a lot of fun to play. Good old Achaz used daggers most of the time, especially after we got the Companion set with its nice, high power mix-and-match powers charts, and he found a dagger +4, returning.
He also ended up with the sword +2, charm person from the Isle of Dread, and occasionally used it to charm a lovely lady or three, but rarely used it in combat. I'd envisioned him as a dagger fighter, and that was how he played. Roleplaying over powergaming, folks!
In coming up with my "perfect" Thief class, I had a lot of options to choose from. Stick with the standard set skill percentages by level model, go customizable like in 2E AD&D, or some free-form system more like d20? Keep the levels about the same, or ramp them up at lower levels and bring them down at higher levels?
In the end, I decided that with a 20 level cap, I'd stick with the original design. Then the choice became which progression to use? Mentzer was too slow, and with a level 20 cap I would have had to revise them to account for 20 levels instead of 36. Moldvay/Cook stops at 14, so I'd have a lot of 99's for six levels or so. 1E AD&D went up to 17, I think, and also had modifiers by race and Dex. Didn't want that level of complexity.
In the end, I did what the good Zeb Cook hinted at in his Expert set, and went with the B/X numbers up until they maxed out, then created superior abilities that can be used at the higher levels. The exception is Pick Pockets, which just continues to grow so that high HD creatures and characters have a chance to be robbed by high level Thieves.
In the end, I went with these 'advanced skills':
Open Magical Locks (open locks), so Thieves have a chance to pick any sort of Wizard Locked or special DM magical portals (yeah, I'd probably allow it for closed planar gates...give them enough rope, and all that). (from 16th level)
Find/Remove Magical Traps (find/remove traps) As above, now the Thief can detect and safely remove Symbol spells, or other nastiness like that. (from 17th level)
Climb Overhangs/Ceilings (climb sheer surfaces) Cook mentioned it, and I felt it would work. The high level Thief becomes Spider-man. (from 14th level)
Remain Hidden (hide in shadows), so the Thief has a chance to make an attack while hiding and not give away his position (from 17th level)
Ventriloquism (move silently) a bit of a stretch, perhaps, but it allows the Thief to communicate with, or distract and confuse, others while moving silently. (from 16th level)
Read Lips (hear noise) so that Thieves can get an idea of what's being said when they're too far away or it's too noisy to hear, but they can see someone's face, obviously. (from 15th level)
Then Backstab, similar to AD&D, gets better as the Thief goes up in levels. 6th level x3, 11th level x4, 16th level x5 damage. Still only ever that +4 to hit, though.
Each of the Advanced Skills starts at a 50% chance at the level it's obtained, and improves from there. So high level Thieves have something to look forward to besides 2 hit points and some small increases to hits and saves after the original skills get maxed out.
Oh, and I went with weapon restrictions as per Mentzer, since that's what I grew up with and what I've always favored (rather than B/X's any weapon is okay, or the short list of AD&D). Any one handed melee weapon, and all ranged weapons.
That's my Thief. Using this class, and judging the use of skills similar to Robert Fisher's famous essay on the subject, I think they should be a decent class to play, and not take away from the dungeon exploration environment.