If we go to the moon (which I'm almost positive we will, since that is literally Zodiark's corpse and where the Ascians live), there might be a 24-man raid or savage dungeon where we will see Magus Sisters/4 Archfiends from FF4. Having the FF1 fiends is tricky because Kraken, Lich, and Tiamat already exist as different kinds of mobs. The 4 fiends from FF4 have more personality and style, anyway. They've nearly existed all bosses from FF1-6, so I think we may start seeing more bosses from FF7+ honestly. Especially with Garlemald being front and center, there's tons of machina in FF7 to draw from. Proud Clad, Airbuster, Heligunner, Carry Armor. There's X-ATM092 from FF8, BGH251F2, and Mobile Type 8