I think Dabi is a highly advanced noumu, like Kurogiri
They said they found a piece of his jaw on that hill. How the fuck could he burn himself to the point to where his jaw falls off and still survive?
Just by looking at how bad his injuries were, he couldn't have gotten out of there by himself. Somebody had to have saved him. The only question is who.
Obviously he didn't go to a normal hospital, otherwise wouldn't Endeavor have found out about it?
Honestly, the only rational explanation I can think of of Toya's "death" is that All for One and Ujiko were involved. My idea is that All for One intended to make Toya a "sleeper agent" for the league of villains. After his accident, Ujiko patched him up, probably gave him another quirk or so and then had his memories of AFO and Ujiko suppressed.
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My idea is that this second quirk was also suppressed along with his memories or Ujiko and AFO. Eventually, through a big fight, he will realize his latent power and awaken the second quirk Ujiko gave him.
I believe that this quirk will give Dabi the ability to reduce his body temperature to subfreezing degrees. He could use this in battle like Dio did in part 1 of Jojo but in practice its real use will come from it offsetting the weakness of his original quirk. Thereby making Dabi easily an S tier villain with strength near prime all might.