Why the lack of FF characters?

[Sephiroth]

With a cast of

Sora and his Nobody fragment Roxas as well as his clone Xion
Master Riku
Kairi
Master Xehanort in all his incarnations (Young, Old, Terra's body, Ansem Seeker of Darkness) and his Nobody Xemnas as well as the other bodies which are connected to him
Master Eraqus
Terra
Master Aqua
Ventus and Vanitas
The Organization and all their Somebodies
Ansem the Wise

we already have a main cast that is important enough and all those smaller bits of Disney and Final Fantasy, except Mickey, Donald and Goofy they only serve the story purpose of just being there to represent a world. If there is more around them then I guess it is cool but it is pretty hard to make a crossover and everyone really relevant enough.