In My Hero Academia, there are few characters as divisive and controversial as Shoto Todoroki's father, the Pro Hero Endeavor. Portrayed initially as an abusive, success-obsessed father and husband who projected his own dreams and insecurities onto his family, it was easy to despise him. Thankfully, this changed by the end of the show's fourth season, by which point he had somewhat begrudgingly become the new No. 1 Hero of Japan, and adjusted his attitude accordingly. Taking a turn for the heroic, Endeavor became kinder, more inspiring and, most importantly, much more deserving of the title of hero.
However, this all might have gone much differently if the character remained faithful to his original concept. According to the MHA's creator, Kohei Horikoshi, his original plan for Endeavor was to have him be a teacher at UA. Horikoshi ultimately decided against this, feeling that Endeavor's presence at the school would make UA "too strong," which would have made certain stories impossible to write. It's hard to argue against the logic, but there's still a lot of potential in the discarded concept.
Many of the attacks against the school wouldn't have been issues at all with Endeavor working there. The USJ incident, in particular, wouldn't have ever happened, and considering how impactful it was to the story it's hard to imagine how much the storyline involving the League of Villains would have been changed. Even the Forest Training Camp arc would have probably changed a lot, especially given Dabi's involvement.
Also, if Endeavor were to be a teacher at UA, it's obvious that his relationship with Shoto would be front and center. At the start of the series, Shoto couldn't stand his father and wanted to be nothing like him. His hatred ran so deep that he refused to use the powers he inherited from Endeavor and relied solely on his mother's, all to get back at Endeavor. With Endeavor being an actual teacher at the school though, it's hard to imagine Shoto having this kind of freedom.
It is entirely likely that Endeavor would have used his position as an instructor to enforce his will on Shoto, taking advantage of the situation and his position. With Shoto spending the entire first half of the show trying to avoid his father in any way he could, this could have been an interesting snag in his efforts. Having no convenient way of separating himself from his father, like he did in the final version, it might have forced Shoto and Endeavor to deal with their problems in a different way.
Without the time and space for Shoto to heal from his trauma, he might have ended up being a much darker and angrier character as a result. Endeavor, in turn, would gain a much more antagonistic presence, one that might not have changed like it did in the final version, which could have created a fascinating dynamic between the students and faculty. There would be a much greater divide between them, with Shoto's classmates and maybe even some teachers siding with Shoto over Endeavor. This would end up creating a kind of contentious force at UA for the students to try and overcome, something that isn't in the series at all.
The later storylines involving Deku, Bakugo and Shoto working with Endeavor at his hero agency would also have had to be scrapped or at the very least completely redone. So far, there hasn't been a single hero to work at an agency and as a teacher, implying that the time commitment for UA is too great to do both. While some faculty are still active and operating, they don't seem attached to any agency. This would mean that UA's new "Big 3" would need somewhere else to go.
But probably the most interesting aspect to this "what-if" would be Endeavor's dynamic with All Might. With the two greatest heroes in Japan working together as teachers in a school, their different teaching styles could have created something of a rivalry. With two distinctly separate philosophies on how to act as a hero and how to approach life in general, another divide could have formed, splitting the school in half not unlike how Marvel Comics' Civil War did.
With all this being said, it is impossible to truly know what would have happened. Horikoshi himself said he felt like this decision would constrain him creatively and for that reason revised Endeavor the way he did. What fans might have gotten would have been totally different, and considering the massive success that My Hero Academia became, everyone involved is probably quite happy with decision.
