While seeing this page of Chapter 82 did it seem to me that Dabi and Todoroki are quite similiar and I couldn't let go of that thought.
First of all are their hair and their eyes, or on Todoroki's case the half of his face that he inherited from his father, quite similiar. The eyes look very alike and the hair as well, with Dabi's being a bit more messy, though the latter isn't really an evidence for either, pro or contra to the idea.
Another thing they have in common is their Quirk and here again is it the half of Todoroki's Quirk he got from his father, the ability to create fire; while Dabi's is a bit different as he creates a darker flame, though this can simply be explained by the idea that his Quirk evolved into this by training it or that he has another mother that changed his flames into darker ones.
Their physical appearance and the Quirk similiarity is something that makes me believe that Todoroki and Dabi are (half-)brothers.
We know that Endavour, Todoroki's father, tried to create a better version of himself by mixing his Quirk with at least one other promising Quirk and that Todoroki is one of many children that is the result of this "experiment" and deemed to be Endavour's first sucessful one on top of that. [1] [2] [3]
It would not be weird if Todoroki would've other older brothers he doesn't know of or if Endavour already tried to create a better version of himself with another woman before Todoroki's mother, with Dabi being an offspring of the two (this might also explain why his flames are different from Endavour's and Todoroki's)
Another thing that plays into this idea is the fact that we do not know Dabi's name. Upon being introduced did he state that he currentely goes by "Dabi" and that he would reveal his real name if it is needed. (Already hinting a connection to something bigger?)
Furthermore do the scars on his body look as if they're from severe burning.
We know for sure that Dabi is considered a villain by society and that he already was one before he adopted Stain's way of thoughts. So he must've done some evil deeds already before his current actions.
If he is now the son of Endavour, then his burning scars might have been inflicted by his father as punishment, either in a fight because of what he did or as a kind of punishment for rebelling against his father and chosing the path of evil.
So I think that Dabi and Todoroki and by that also Endavour are related to each other and that this might play out to be interesting in the future story.
