(here's hoping I won't be burned at the stake)
From the B/X Companion, Part 2: Player Character Information:
Many Master Thieves retire from adventuring, content to run their hideout and live off the wages earned by young apprentice thieves. Others continue to hone their skills past the normal level of mastery until their abilities seem almost super-human.
Most of a thief’s normal abilities (open locks, find/remove traps, climb walls, move silently, hide in shadows, hear noise) cease to improve past 14th level though they continue to function as per the Basic rules. Neither does the thief improve at reading languages or magic-user and elf scrolls. A thief’s backstab damage DOES improve: at 16th level damage is triple normal, at 24th level it is quadruple, and at 32nd it is quintuple (5 times normal). The “to hit” bonus when attacking from behind does not improve at higher levels.
Maximum level for a human thief is 36th level; thieves continue to gain 2 hit points per level after reaching Name (9th) level.
The following new skills are available to thieves above 14th level that have mastered all earlier skills:
Craft Device: this is the thief’s ability to construct elaborate traps of mechanical nature. Cost and time to construct will need to be decided by the DM (similar to the construction of magical devices). Thieves use these devices to protect their hideouts, though they may build them for others at a price. Failing the craft roll by more than 10% means the device was not constructed correctly, and all time, money, and components are wasted. Failing the roll by 10% or less indicated the thief successfully created the device, but is himself the first victim of the device as he sets off the trap!
Physical Prowess: the master thief’s continuous training and physical conditioning allows him or her to perform amazing feats of agility. A successful roll will allow the thief to climb an inverted overhang, balance on a tightrope without apparatus, somersault over an opponent in melee (to get behind him), or perform a safety roll to avoid half damage from a fall. The DM is final arbiter of what is possible (for example, “rolling with a fall” may not save damage from a spiked pit), but most physical stunts should be possible with a successful prowess roll.
Deception: the master’s disciplined ear and study of languages, as well as control over his own body movement, allows the thief to disguise himself, mimic speech and mannerisms, and even throw his voice (as the 1st level spell ventriloquism). The DM should roll whenever the thief attempts to deceive someone; the thief will always believe his deception has succeeded. If the roll fails, the thief’s opponent will know that the thief is not what he appears to be (or will know where the voice really came from, in the case of failed ventriloquism). The DM will determine the reaction of the presumptive victim.
[Design Notes: the tables for the thief are not included here due to my inability to scribe tables with any degree of accuracy in html. However, all new abilities begin at 36% for level 15 and increase 3% per level up to 99% at level 36. Pick pockets continues to increase 5% per level after 14. All other skills, save backstabbing damage, are the same at level 15 or 36 as they were at level 14. I wanted to build on the original B/X rules, not revise what's been written (as Mentzer did with BECMI).
36% may seem high for the starting level of a new skill, but I want PCs to have incentive to try the new abilities right away. Also there may be rules in the DM Section regarding using these skills at lower level (basically, adding together multiple "normal" thief abilites, and dividing the average by 3. For example, crafting a fiendish device at 14th level would mean dividing the average of Open Locks + Find/Remove Traps by 3% = 33%. All new abilities are thus built firmly upon the foundation of the old.
Personally, I think this returns thieves to their "adventurer" roots rather than making them swashbuckling fighters. I also wanted to give them abilities that really distinguish them from what other classes do...as well as make them interesting enough that I would want to play one at high level.]