About the Dabi-is-a-Todoroki theory
Okay hear me out on this
You can easily tell that Shouto Todoroki's character could be influenced by Zuko from Avatar The Last Airbender. This is nothing new, they share a big, big load of similarities, and the mangaka, Kohei Horikoshi, is known to be a fan of western hero culture, so it's entirely possible and dare I say, very likely that he watched this show as well.
Now, assuming that these two characters resemble eachother, there will surely be more parallel traits they share.
What about siblings?
Obviously, Azula isn't anything like Fuyumi. She's far too nice, it makes absolutely no sense for this to be it. This leaves the question of, where is Azula?
And then there's fucking Dabi.
Blue flames, anyone?
Maybe this is just me overthinking this. There are already theories pointing out Endeavour's blue flames, the iconic blue eyes and burn marks. I'm just adding my own part to it all.
If you want to dive deeper into this, I was thinking that maybe Dabi was the one who experianced what Zuko did, while Shouto is in Azula's position, despite their personalities matching the opposite position. It's a switch around, but could be possible. It is true that Ozai viewed Zuko as a failure like Endeavour does his other kids, so who knows.