I think they're already in there. The four bots that go megazord at the end each have elemental colours corresponding to the four fiends. I won't be surprised if they have the same names.
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I also think there's some kanji on the zords that might point towards them being our "fiends"
if they waste the 4 Fiends on 4 bloody robots, I'm going to riot. Seriously, I hate this cyborg/robot stuff, but that's just an opinion. Still, turning 4 interesting bosses into robots would be silly, imo.
I think right now they're more on a FFVI kick. Alexander was first introduced in that game, plus Sephirot was one of the goddess statues in FFVI as well, and we can pretty easy say that we'll be getting the other two goddesses from VI in the upcoming patches. Four fiends would be awesome, but probably not till later in the game.
Check out the original Amano design for the Giant of Babil:
Alexander is widely believed to be based on the Giant and the Proto-Babil (a smaller prototype version of the giant) superboss from the FF4(3DS) remake uses Divine Judgement (Alexander's traditional summon attack). The idea that this mad scientist goblin is a Dr. Lugae reference isn't far fetched.