The next Avengers film is meant to be the end of the contract for most of the cast. Some of the actors have stated they are not bothered about continuing with the roles. Some may want more money to reprise the role. So they will be losing actors/characters.
They will need to introduce new lesser known characters, and these may not be for everyone ir not everyone is familiar with them
The superhero bubble may burst. Everything as a time and then genres fade i.e. western films
Disney buying Fox. If Disney buy fox, they will get Fantastic Four which could do with a reboot (again) and X-men. People like the xmen films and know the characters and actors, plus they have done a great “reboot” movie in Days of Future Past, so would fans want to see another actor play Xavier, Magneto or Wolverine etc again and redo and force the xmen into a reboot. That goes for Deadpool as well