To be honest, as a long time game master and allround roleplaying geek, I do think I would prefer crafting a world from scratch to adopting one that is already deployed. Aside ensuring that the existing players are not disturbed it allows for a story telling experience with fewer surprises.
I have been doing the same basic thing for pen and paper games for years, it would be fun to translate some of that to a community project that defines a unique, believable world for players to interact with.
Mostly I think it would be fun because we get to create something unique and wonderful. Something better.