The trailer has just been released and everybody's already freaking out about who this girl may be. I have a few possible answers for ya...
Shuri - Black Panther's Sister.
She's obviously African-American (if she's human) so this might not be too far off, especially with Black Panther's movie coming up.
Maybe she's some sort of Inhuman emerging from the movie version of the Terrigen Mists? Not too far off as well since the Inhumans also have an upcoming movie.
Ultron's lover and Avenger's member.
Guardian of the Galaxy, Avenger. She can alter mental traits such as personality, as well as take control of one's mind.
This is very unlikely but anything's possible I suppose.