The fans are the main driving force, but the moderators supervize everything, so all new entries/old edited posts will be reported to the mods(via email or some in-mail system), and they take a look at it and either approve or delete telling this is not good enough/needs editing.
New categories are created by mods - fans can only edit existing categories. Anyone at all can edit, but like I wrote above, moderators approve/disapprove of everything written.
This basically means that you need very active moderators who know how actively check out what is correct and what is not, and what is scientific and what is simply I'm-12-and-want-to-write-on-a-website.
As for categories, hmm, that's a tough one. Initial categores should contain factions, not only definitions but how to deal with factions in the best possible way, character development(perks, magic, etc), a main quest walkthrough with the different branches. Of course you need sidequests too, but I'd divide those either in factions or cities, probably cities as that's how they are categorized in the game. Maybe there should also be a technical directory with possible tweaks and such to get the game running as smoothly as possible, since it certainly has quite a few technical issues right now(inspired by the huge tweak thread we have now, something similar as a wiki, but more organized obviously).
Well, those are my suggestions at least.
Edit:
@MonGoliat:
A wiki is something similar to wikipedia.org, an encyclopedia where fans can contribute instead of just proffesional writers. So basically, you can get huge amounts of information(about Gothic 3 in this particular wiki) in very little time, but the information needs to be moderated so it doesn't get out of hand or contain false information. Basically it's a very thorough and solid FAQ, written by fans who know what they're talking about.