I can tell you my opinion is that would be a interesting concept to go with making mystique secretly working for apocalypse all along while biting hard time until her true master returns to create destruction. very interesting I must say. soul mystique popped up in Xavier kitchen as not a little girl but as a poser to get his sympathy and slide right into position. then leaving Xavier to team up with magneto because she missed causing havoc which apocalypse is all about that could be the reason why she also teamed up with magneto until the return of apocalypse. We will see.