Everybody loves a Thief, so I hate to dilute the class into merely a Chaotic Fighter. A lot of interesting adjustments were discussed on the OD&D board I visit (the same that hosts FIGHT ON magazine topics). That is where a d12/2d6 roll for skills comes from, among other changes. To notate the change from the original Thief class, I'm calling this modification the Knave class:

KNAVE

Prime Req: DEX

Restrictions: Knaves use d6 to determine hit points, and can use the following weapons: dagger, club, dart, javelin, bows, hand axe, falchion, broadsword and whip/weighted chain. Knaves can only perform their special skills (see table) in non-metal armor and without a shield in hand. Otherwise, they are only prohibited from wearing plate. All hidden characters can attempt a surprise backstab if undetected, dealing maximum damage per weapon. Knaves cannot be Lawful, or refuse membership in a Thieves Guild/Clan after level 5.

Special Skills: Knaves following the armor restriction and acting unencumbered or without a serious wound can advance on the table below. A d12 roll that equals or exceeds the target number indicates a success. DMs can increase this number for situational difficulty (like a complicated lock). Knaves also have a "dirty fighting" skill that allows them to perform a secondary attack against a melee target reduced to 1 HP. This "assassin move" strikes for one point of damage, slaying the target.