[quote=VitaVitaAR]
I'm... still trying to think of challenge ideas that wouldn't be a breeze for some people and terrible for others. ^^;
[/quote]
I was thinking of taking the 'separating the party' idea and turning that into a gimmick. Let's say we have a dungeon with two or three different paths. You could go down one path, but up to a certain point you couldn't proceed without intervention from another group on another path. Think of Kefka's Tower from Final Fantasy VI.

Also, it's pretty clear that every character here has some sort of power or basic thing that's absolutely unique to them. Perhaps certain challenges would revolve around the use of such character's abilities: for instance, Sazh's piloting skill, or Nonon's musical guns, or Felt's alchemy knowledge. We could do multiple such things, even simultaneously, or have them coordinate, or have others protect those important guys.

Just throwing out inspiration, I don't actually have any specific ideas myself. At least, not any worth sharing.