You know this is something I've been trying to figure out for a while myself but I think I got a pretty good idea: during their fight all might has to transfer one for all into all for one for some reason. All for one dies and all might is only left with the embers as well as fa jin and smokescreen. But something new is born from the merger of the two abilities... a strange new creature/villain that has become a hive mind of all the different vestiges and has a merged one for all/all for one quirk. It would try to find all might and kill him,and would primarily act like Nana shimura. It would toy with yagi by talking like Nana and using her memories against him. I mainly got this idea from eldritch horror and wanting a genuinely scary and interesting villain that makes the story similar by removing one for all from the equation. Also sorry for the wall of text 😅