Vegnagun would work like the ansem 3rd form (kh1) did. YuRiPa, I'd like to see as opponents in the coliseum. Sephiroth looks cool, but i'd like to see a sidequest involving helping him hunt down something (a la advent children) then right before sora gives said thing to sephi cloud intervenes, yadda yadda yadda...

hey! what if there were Sin-heartless hybrids?! That'd give sora a run for his munny!