If ratings are strong, they won't let the 13 regenerations thing stop them from making more. Off the top of my head:
1. River could've given him more then we realize. He expects to die after his 13th life and he doesn't and sits up amazed "Well that shouldn't have happened"
I like that one and it would fit best into the show.
Think about how ever regeneration could be the Doctor's last because he just doesn't know how many are left. I think that would be great for the series because clearly they will never actually end it after the 13th Doctor.
But I do like the idea of ending it and they can do spin offs and such with his daughter or soemthing. be kind of fun.
I also think that this is the best case scenario ... and it would make a great suspense builder because he would not know how many are left him in the future.