Theories. We know that Paramount has the rights to use Quicksilver in upcoming Avengers films, so if you have Quicksilver, why on Earth would you not also push for Scarlet Witch as well? I doubt the X-Men universe would want anything to do with her.
Anyway, theories. Coulson died during New York. Coulson is alive again. Coulson is being cagey about what happened, but every time someone mentions Tahiti, he says, "It's a magical place." Now, either he's a LMD and that's just part of his programming, or he's making a subtle reference to ACTUAL magic that brought him back from the dead.
And since the Internet loves its conspiracy theories, let's not forget that Wanda is in love with a certain android.