So since this is apparently a Dissidia raid...

[Nihility]

I totally expect Sephiroth if they want to go full fanservice.

I would expect a "7" dungeon would more likely be Jenova themed

the 4 fiends or the moon weapon guardians would also make for a good theme(though i'd personally hope for the 4 fiends to be a series of trials)

1 has a nice "4 fiends" as well

3 and 6 have already been somewhat stripmined so i'm struggling to see anything else as perfect as using the dimensional castle bosses from 5. I guess they could just use random stuff like Doomtrain and Hidon