After Sephiroths death, and after all Weapons being killed, the planet realized how bad the humans could be. They slowly destroy the planet, they destroy the nature for the other animals, and they even kill themselves. So the planet waited five years, and while it was waiting, it created new Weapons. New ones, even more powerful than the ones who previously defended the planet. Their only purpose is to kill all humans, and all traces of them. The planet isn´t happy before the smallest little trace of their existance have been crushed and taken back by the nature.
In this plotline all your characters is much much stronger than now. Now you may have your strongest limit break, strong summons like Bahamut ZERO and even Knights Of The Round. Materias like Quadra Magic, Mega All and even Master Materias can be used(allthought it isn´t allowed to be too strong, like all Master Materias with Quadra magic and things like that).
The new weapons isn´t made yet, althought I have a name for one of them: Death Weapon.