Technically the first one to use magic would be Loki.
I don't know. I hope the Marvel U doesn't try and use too many M Night Shymalon type twists all the time. It worked really well for Iron Man 3 but not for every film.
Of course, the twist might be that after a few months Whedon and Feige decide they can't fit in Wanda and Pietro after all and we're all left like