New Beginnings

With only one, small city remaining as civilization in Japan, the last of the UA heroes come together to enact their last remaining plan for success. Deku is elected as the one to be sent back in time to save the future, to stop a world overrun with Noumu and an overpowered, vengeful Shigaraki. He is, however, sent back a bit too far - 6 years too far infact.

OR

The one where the world is fucked, Izuku becomes a 3 year old and everyone becomes a hero.

Also on Quotev and AO3 under the same name!