Your perspective is super interesting to me because I'm extremely familiar with the common shounen pitfalls you're mentioning, yet we have almost the exact opposite impressions on how these are being handled in MHA.
I think you believe that if an issue isn't wrapped up within that arc then the problem doesn't exist anymore, isn't being addressed, and is never coming back - almost like "out of sight, out of mind". For instance, it seems like you believe that since Horikoshi didn't go through Endeavor's past or in-depth about Hawks now, those things are never going to be resolved. You mention Todoroki "fizzling out" after the fight with Deku and Stain.
I see things the opposite way: this is going to be a LONG series - you can't give too much character development at one time. Frankly, people themselves take a while to develop and resolve their issues. Example: We see Todoroki use his flame side against Midoriya during the sports festival, something he'd refused to do because of his feelings toward his father. He then continues to use that flame technique in following arcs, but there's no further development. Then suddenly, with the introduction of Inasa Yoarashi, Todoroki has to come to terms with the realization that he ISN'T completely over the feelings toward his father, that he can't just forget his lineage but has to find a way to accept it. This dilemma takes place with huge stakes: he's facing down a Top 10 ranked hero and his provisional license is on the line. Through this encounter, he reaches another stage of acceptance and unlocks the ability to use both ice and fire abilities at the same time.
This progression takes place over the course of multiple arcs, during which many other heroes and students are highlighted; yet, once we're in that provisional license exam, the stakes still feel real and you root for Todoroki. Horikoshi is able to take a character that hasn't been in the limelight for a minute and still make them MATTER. Expect the same for other characters that have been introduced, for mysteries like OfA's true power and Dabi's lineage, etc. It's how you have to build a story that may last for 15-20 years - word to One Piece.