Sephiroth would work as a final boss since has multiple stages (which by now is relevant as it needs a savage only mode) but i think its much more likely omega itself is the final fight perhaps it could have a ff7 moment to pay homage.

A lot of iconic ff7 bosses are already in the game ... not that it has a lot now that i think about it a lot of them are generic monsters. The weapon types are already in game, so is guard scorpion.