I'd like to dispute the theory that Hisashi Midoriya is secretly All for One, on the grounds that tying Izuku Midoriya's bloodline into the All for One vs One for All conflict would mean Izuku was born special. This twist would contradict the idea that Izuku is special in spite of the circumstances of his birth and because of his own determination, effort, and actions.
I've heard people argue that that the Star Wars allusions and references should be interpreted as evidence to support the "All for One is Hisashi" theory, but I disagree. I think we already got the My Hero Academia version of "no, I am you're father" in the form of Tomura Shigaraki being revealed as Nana Shimura's grandson.
It's certainly possible that Horikoshi could reveal Hisashi and AfO to be the same person, but I don't think he's going to because it would be bad retcon and an unneccessary one.