When Dabi popped up in front Hawks and Endeavor (which I think was a spurr of the moment thing from him, like seeing Endeavor in the state of an easy prey egged him on to just appear and go for it) I realise what a massive hit to the whole hero system it would be if Dabi revealed himself to be one of Endeavor's abused children.
And right after he does that the League launches whatever massive attack.
I'm not sure if Endeavor would die get incapacitated or just seriously injured but ultimately ok after that revelation (it would probably be at the end of a big fight). And I'm also not sure which one I'm most interested in.
It would also be an major revelation for Shoto who probably doesn't even remember Touya that much. And I can see him becoming really focused on saving and/or bringing Dabi in. This is, as you say, a living breathing representation of his father's sins, a serial killer, family member, possibly the person that can relate to Shoto's past the most and something he will absolutely have to deal with. The sooner we can get all of that rolling the better.
Final side note. In the latest chapter, when Endeavor mentions that Deku is one of the people that can hurt themselves with their quirk, "One of us.", I wouldn't be surprised if he's remembering his lost son who had greater fire powers than he did but also Rei's incompatible cold based constitution.