As much as I would like to have The Rani return, I hope Missy isn't her. Not just because the mental gymnastics that would have to be performed in order to make sense of her behavior for the last three seasons pretending to be The Master, but also because it would rob the power of seeing a major Time Lord character change gender. As oppose to hearing about it (The Corsair) or possibly a one-off character (Donald Sumpter's The President who may or may not be the resurrected Rassilon).