I'd prefer the cross-overs to remain in-house, or at least within the same... genre. If not other SE games with SE characters (part of me would love to see Geno and Mallow from Super Mario RPG: Legend of the Seven Stars...), then at most a Kingdom Hearts Cross Over. Anything beyond that feels like it'd be pushing it. Don't get me wrong, I'd love to see other little cross-over events with JRPGs (Suikoden Crossover...) but, it just doesn't make a whole lot of sense with SE either owning the game and property, or the characters within it.

A DC cross-over would feel silly and forced and completely irrelevant to the Final Fantasy Franchise.