Re: 50th Is "The Day Of The Doctor" (And Is 75 Minutes)
The Timelords gave the Master a new set of regenerations during the Time War. So they've already set up that it is possible to bypass the regeneration limit using some Time Lord thing, they just need to write it into a script that the Doctor finds it. I doubt they'll have him take over someone's body due to it not being something the Doctor would do or have a new character who claims to be the Doctor. They can't even get that to work in comics.
Awesome Possum is in the Burn Book