And these are characters that will likely be tied to the main story beats. The Twins are used by Hydra to attack the Avengers and keep them busy while they complete their Ultron Imperative. Ultron goes rogue, builds The Vision, and sends him to Wakanda to obtain Vibranium for his upgrade. Fury sends The Avengers to Wakanda to assist The Black Panther and his troops.
I don't think Ms. Marvel will be introduced, but a Carol Danvers cameo appearance could happen, similar to Hawkeye in Thor and
in The Winter Soldier.
It's really not hard to juggle this many characters when they're tied to the story for very specific reasons. Unlike when other studios try to pack in a bunch of villains or something that have minimal ties to each other.