ffsjustgimmeaname
Shigaraki never planned on becoming a brother, he was happy as an only child. Except one night when he's walking the streets of Musutafu at fifteen, he stumbles into a toddler with now where to go.
He was just gonna take care of the kid for a few days till he could find somewhere to dump the child. Ten years later he still has the kid. And in between his full time villain career, he has to be a brother and a parent to a loud explosive teen with homicidal tendencies. Oh and, has he mentioned that said teen is applying to UA?
Or;
Shigaraki adopts Bakugo, who becomes very good at being a villain, but still strives to be the number one hero.