I get your concerns. But this sequel wont need to spend a big part of the movie presenting characters, like FC or X1. So the very first scenes on the future and past will go straight to the main plot. With the 60's x-men already formed, on the school or wherever they are in the very beginning. The development of the main characters will be during that hour and some more minutes. This wont be a conventional sequel where there is only one x-men team and the 2 hours will be used to show all the things each x-man does. This is a two-timelines sequel, so we cant expect the same treatment to any of the two casts. This is a crossover, so as such we have to watch it. The next team movie will have more time to develop better each character.