inb4 crono-cross Lavos/Time Eater appearance.

Seriously a Sephiroth appearance is too far fetched. Even if Cloud Appeared in this world already from FFT which both Eorza and FFT are tied to Ivalice, doesn't explain why Areith was were as part of the quest. Likely just fan service and nothing more.

As far as Major Boss fights in the future, id like to see Diabolos, Gilgamesh, Cuja (FF Boss Fight from Super Mario RPG), Emperor, Doom Train, Omega Weapon.
For minor boss fights in new dungeons/ hm dungeons, id like to see Materia Keeper(FF7), Anima (FFX), Zeromus, Hades,

Id also like to see Ruby Weapon return in Sagolli Desert as part of the Gold Saucer Patch.