Quirk theory
I'm absolutely exhausted at 4:00 AM I'm going to bed once I post this but this has always been in my head for like two years I just haven't written it i'm sorry if it is kind of hard to read or understand
OK hear me out I'm giving a little head Canon I have ok my theory you know how Midori is quirkless even being quirkless they still get somewhat of an immunity from their parents so you know how Midori's parents are telekinetic and Fire breathing I'll just use the word dragon anyways wouldn't It give him some fire resistance considering that when he uses one for all in the show he has lightning which would just be a mix between the two quirks added to that but he doesn't actually have the quirks to go with it it's just a side effect and then for all might he doesn't have it but he does have it where he's just like transform into the hulk kind of thing so there he is that one of his parents or both had something to do with things Mount Lady changing size enhancements I don't know something like that
This is going off of the theory that all for one is not the father!!!
Bob Goodnight