Thieves' World! What a great idea, and so well suited to pbp. I think that it should be a number of small group threads, some of them one on one, unless you really want to encourage real story-writing in this. That could be a good thing, but it would make for very long posts and probably a very long time between posts.
The pitfall that I can see is that one writer, or set of writers, could have to wait a very long time for another group to be ready to have their intersection between the two groups together. That would have to be carefully coordinated. It really might work better in the story section than in pbp.
I read the first several Thieves' World books, but some of the material in the series, particularly the later stuff, is very hard to find. If we do this, and we set it in the same Sanctuary, it should be after the one of the first three books and not much later on.
And, you know, some of the writers from those stories might be very much at home here too.