It's not exactly a pressing issue, since I won't be running Thieves' World anytime soon. I want to familiarize myself with the source fiction (and not just the RPG material) before I begin, and I have other gaming plans in the near future anyway. I'm also completely spoiled for choice; the box set includes NPC stats for nine different games, and some later publications (like the Thieves' World Companion and an article in Different Worlds #19) add a few more games to the mix. Besides, most of the other setting information in the box set is presented in a system-neutral manner, so I guess you could conceivably use it with almost any system.
Long story short, I made a list of game systems I'm considering for use with Thieves' World. Nothing definitive, just some ideas I had while the topic was on my mind. Here's what I've got so far, in no particular order:
- B/X D&D, with or without either my BX+RC bridge rules or my BX25 house rules.
- AD&D 1st Edition, with or without the additional rules from TSR's Lankhmar box set.
- The Fantasy Trip
- Tunnels & Trolls
- Stormbringer (probably 4th Edition)
- Neoclassical Geek Revival
- The Black Hack (or one of its 10 bazillion variations)
- Into the Odd
- Maze Rats
- Dungeon Crawl Classics
- Original Edition Delta
- Low Fantasy Gaming
- Dark Fantasy Basic
- Astonishing Swordsmen & Sorcerers of Hyperborea
- Warhammer Fantasy Roleplay (probably 1st Edition, possibly 2nd or 4th)
- Traveller (suggested by Kelvin Green in the comments)
P.S. If I do end up using some variety of D&D, I'm also tempted to throw in a pretty crazy house rule. Originally, I was going to make all Blue Star Adepts immortal as a class feature. But what if everyone of a certain level or higher (let's say 9th level, the fabled "name level") were to become immortal, PC and NPC alike, regardless of class? My reasoning is that the gods are very active participants in the setting, and that anyone who reaches such a level of skill and importance is bound to be chosen by one or more gods as a servant or avatar of sorts, a pawn in their machinations, whether they know it or not. This wouldn't exactly be a known fact among the mortal populace, except insofar as they know that sometimes important or legendary figures tend to catch the gods' attention, which can have a variety of effects for good or ill. But if you become an experienced enough character, you're going to catch the attention of someone or something that will want to keep you around for a while as part of some ineffable scheme...