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"
2. The "507" lives is canon, so, no explanation bothered with
3. 13 Finds/Meets a Dues Ex Machina that gives him a new Cycle
4. Time Lord Society comes back long enough to give him a new Cycle
5. Maybe someone who's People he saves, somehow transfers Life force enough for more regenerations
6. He somehow absorbed life from The End of Time brief Time Lords return
The explanations can go on and on