It sounds fine. It's certainly starting at a better place than Warcraft which required a brushfire to clear out enough space to tell a more coherent tale.
That sounds like the perfect opportunity to insert something without worrying about purist complaints or continuity nonsense. It's not like this needs to be another adaption. An independent tale of an idealistic pixar-esque superhero team up doesn't seem that difficult considering the current environment with the MCU, an Incredibles sequel or even the fairly forgettable Big Hero 6.
As for the cast, meh, not everyone usually gets to be in a first film. That's just what happens with any sprawling ensemble.