My prediction, is after Capaldi, will be the female doctor. Possibly a recharge of 12 female lives. Then, if fans want a male doctor back, the female doctor(s) can always team up with McGann, Hurt, Eccelston, Tennant, Smith, and Capaldi for as long as they live.
Yeah, they opened that door in Night of the Doctor, however it depends if the BBC wants to be that risky...