Not really liking this idea. Wanda and Pietro are the children of Magneto, and are themselves mutants. That's part of the core of their characters, it's who they are. Obviously they can't have any of that in the movie, so the characters would have to be drastically altered to fit them in.
They did something similar with Juggernaut in X3. They made him a mutant (yeah, I know he is in Ultimate, but not traditionally), and it seemed to ignore his relationship with Xavier.
agreed. if they are gonna bring in some more characters, I'd much rather see someone like Vision, and eventually Ant-Man and Wasp.