Ok, so I started to watch BNHA about 2 months ago and I watched up until the end of season three, waiting for season four. I want to read the manga, but not right now. I have multiple reasons why Todoroki and Dabi are secretly brothers. I know many people believe this is true, but hear me out.
Ok, so the first thing is, Do we know how Dabi got his scars/patchwork? No, not really (keep in mind I have never read the manga, I will soon, but don't come at me for not having the correct information) Maybe it is because Endeavor punished/hurt him for not having a strong enough quirk for his liking. He thought it was not strong enough to surpass All Might. That might be why Dabi ran away. My friend and I were talking and she said, "Well, maybe His mother did the same thing to Dabi as she did to Todoroki." I said that wasn't possible since Dabis scars and Todorokis scars look nothing alike. Todorokis scars have a more reddish tone, while Dabis scars have a more purplish, blackish tone. Although those could be from his quirk when he was younger, I seriously doubt it.
My friend also said that "Maybe after Todoroki was hurt, Endeavor thought he would be strong enough afterward, so he chose to work on Dabis strength. It could have been flipped around and Dabi was jealous of Todorokis power, so he became a villain and received his power from Tomura's sensei's All For One." I said no because Tordoroki is 15 and Dabi is 20 - 23 years old. Normally your quirk develops around the age of four. That couldn't have happened because Todoroki would have remembered The time, but he didn't
That's all I have for now. Bye!