Beast Bonded Witch is a very nice pre-existing ingame mechanic to accomodate the Time Lord's ability to regenerate. There's a druid archtype that permits the same thing too using the reincarnate spell as the capstone but it is nowhere near as cool. Shame it doesn't stack with the Dimensional Occultist archetype but you can pick the Time Patron at least. INT based too. Probably better suited to The Master than The Doctor though (body snatching is definitely his MO).