Put your concepts you'd like to see in the comments!
1)I'd like to see a more in-depth look into the Quirkless community and how they're treated by wider society. It would be interesting if we see a Quirkless hero or villain emerge, forced to be very tactical to keep up with others.
Specially with a Villain, I can envision them putting plans/analysis on the internet for other villains to exploit heros' weaknesses. Or, they exposing Heros' private information, such as Endeavour's abuse towards his family, ruining his career and possibly getting him arrested.
2) I'd like to see the Quirk equivalent of mental hospital/rehabilitation, people who have uncontrollable quirks posing a danger to themselves or the public but isn't genuinely their fault. How would the My Hero Academia world handle a mutation quirk constantly giving off deadly radiation?
3) Going off the pervious idea, what if there was a Quirk so extreme, the user would accidentally kill most people who walked within their general area. However, the user themselves are in great pain and live in total isolated. This puts the Pro Hero's no killing rule into question - If they were the only ones powerful enough to kill this unstoppable force, would they break their rule? Would it be better to keep this Quirk user alive or dead?
4) Is there a Quirk that allows the User to permanently transform those they touch? If so, I can imagine a scenario, where a Corrupt country, uses this to their advantage. Instead of going to prison system, some Criminals are permanently transformed as their punishment, literally losing their humanity.
5) Can certain Quirks gain a consciousness seprate from their user? For example, if Dark Shadow decided to be the domiant one instead of Tokoyami *Cough*Traitor?*Cough* I can imagine this being a way the quirk singularity rising - That conscious quirks want to seprate from their human users.
6) Are there permits that allow people those outside of heros, to use their quirks? It seems a waste to not allow someone with a healing quirk, not to use in a hosptial.
7) We've seen Heros with sterotypical villain quirks, but what about a villain with a heroic quirk? Imagine a villain with a healing quirk who doesn't like working with others.
8) We've seen ideals like Stain's, who wants Heros to kept to massively high standards. Though what about the opposite, a hero who's sick of Pros being idealized like gods and given impossable expertations. Instead, they want Heros to be treated like careers such as Policemen. Prehaps they don't even like the label "Hero", thinking it undermines others hardworking people like Doctors.