Whedon implies Scarlet Witch and Quicksilver to Avengers 2!
“I’ve done enough shows and movies to be comfortable with that. You do come to a point where we all agree. ‘Okay. Oh, I’ve got these two characters, two of my favorite characters from the comic book; a brother-sister act. They’re in the movie. That’s exciting.’ You know, you lock certain things in”
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.