I'm also picturing something sort of along the lines of something cross between Urban Dictionary, LJ, and Slashdot, with a little bit of E2 thrown in for good measure.
Entries: in alphabetical order, with Urban Dictionary/E2 style definitions. Comments, LJ style. Ratings on comments and definitions, Slashdot-style, with /. style threading, even, perhaps. E2-style related links at the bottom. E2 node-ranking on the entries overall. Sweet, flexible, internal search engine.
Hell, that would be good for anything, not just a fandom dictionary thing. Any thing where you've got an environment with things that need defining, and multiple users doing it, and people who want to comment on it but not define, and enough people doing it so that you need to sort out the crap from the actual content. You could even restrict people who were allowed to make definitions -- only certain users, all registered users, or even anonymous users.