If Sephiroth had left Shinra, part of me wouldn't really want him joining up with Avalanche tbh. I think he would certainly sympathize with Avalanche and probably would be propositioned as a candidate to join. But I honestly think that Sephiroth probably would just want to live the rest of his life in peace not being a pawn in someone else's game. He's tired.

Sephiroth needs to fuck off into the wilderness and become a local cryptid. Maybe he protects a local village from danger but he doesn't interact with anyone except the local brave kids who dare each other to enter the forest to find the silver beast.

Sephiroth would likely live an extended life too due to his Jenova cells. So he'd become an urban legend for several hundred years lmao like the Jersey Devil

Sephiroth is the type who just wants some peace and quiet. He'd spend a lot of time just resting and enjoying nature. He'd connect a lot with animals, but if the world were to be kind and embrace him, he might give it another shot in integrating with society. But it would definitely take years to undo a lot of the trauma that's been done to him.