For whatever reason, using actual protagonists/antagonists from the franchise really bugs me. I'm not sure why I draw the line where I do (and it's not even consistent; I have no issue with ExDeath, for instance), but the idea of fighting Cloud or Zidane or SIN or Ultimecia makes me roll my eyes and pray that doesn't happen (that said, I like the the idea of the Four Fiends, and they would certainly fit the raid structure nicely).
It's also very difficult to speculate future bosses with the Full Franchise approach, as they could literally be anything. Assuming that the game is going to stick to FFV, however, makes speculation quite a bit easier. This is actually pretty likely; not only is the setting itself, as a whole, straight out of FFV, the primary antagonist (Omega) uses its FFV appearance, and I think it's a pretty safe bet that we'll be fighting Omega as the final battle.
So what other FFV bosses are likely, given that the final boss of the game has been used already? Well, half of this current tier (Catastrophe and Halicarnassus) are Demons of the Rift, so that would be a likely source of inspiration for the other fights. Melusine, Calofisteri, Apanda, and Twintania are obviously already elsewhere in the game (although that didn't stop them with Halicarnassus); that leaves Necrophobe, Azulmagia, Wendigo, and the Triton/Nereid/Phobos trio. Boom, full tier right there. Gilgamesh is also a possibility, as is the Magissa and Forza duo, and as some have mentioned, a fight against a heroic character isn't entirely out of the question. The final tier, then, could be Gilgamesh - Forza/Magissa - Warriors of Light party - Omega.
There's also the possibility that we'll see Shinryu again; given the relationship between Shinryu and Omega, it seems strange that the only appearance they have together is a quick 3.56 cutscene, especially when the latter's raid series references FFV so heavily. On the other hand, we already have an 8-man fight against Shinryu, so that might be a bit redundant to include in the raid as well.