I'm a big fan of large, sprawling fantasy narratives that involve lots of interweaving threads that eventually begin to spiral together into a tightening knot. However, something that complex requires a lot of forethought, and several people being on the exact same page--it might not work out very well. Not unless you have several very compelling 'centerpieces' everyone can agree on and work with (things like Wild Cards worked explicitly because of that notion).
Something a bit sillier and more fun might be a collaborative superhero piece; there's a lot of room to play without necessary screwing up someone else's narrative there (and superhero settings tend to have enough elasticity to allow for tons of nonsense anyway--the medium is used to multiple authors screwing up each other's storyline). Another alternative I like involves something akin to Planescape's setting, involving a city at the center of an immense multiverse, allowing for a vast variety of ideas to conflict and intermingle.
I think the most important piece of making collaborative projects like these work is getting everyone to agree on one or more 'centerpieces', and working out from there--allowing a flow of creative, individual ideas to extend out of that centerpiece. Otherwise, you end up with a lot of fractured ideas with nothing to attach themselves to.