How about this idea:
The universe won´t let the Doctor die.
Simple as that. When he rebooted the universe at the end of season 5, he became linked to it. He´s now an integral part of the fabric of time and and space itself. The universe will always find a way to bring the doctor back. Not nessecarily via a 13th regeneration right away. He would be reincarnated the classic budissm way, when enough linear time has passed (lets say...like...twenty years) the TARDIS would materialise around him and he would get his memories back. And this would start a new regeneration cycle.