In the worst possible scenario at the end of Dirge of Cerberus, Vincent Valentine receives the unexpected chance to return to the past to change the events that occurred and save the planet from its total destruction, while at the same time discovering truths about himself that wrongly have been concealed from him. (Vincent x Lucrecia - later Sephiroth x Aerith) (Timetravel/AU)
Self-Insert fic. An adult in a four year old Sephiroth's body can't do anything about capitalism or ecological collapse, but deliberately being off-putting and weird to antagonise the people complicit in your circumstances is a decent hobby if you need to pass the time.