I've been watching classic episodes in order to gear up for next week's BBCAmerica takeover, with the return series airing in order. I've watched some pertinent ones like "Terror of the Zygons," since they will be in the 50th, and "Underworld," because it was the first ever I remember seeing thirty-something years ago. Right now I'm watching "Mawdryn Undead" simply because it's one of my favorites.
The thing about the last two I mentioned is that in both of them The Doctor had encountered races with whom the Timelords had shared regeneration technology as well as having it stolen from them. All The Doctor needs to do is visit the Minyans again if he needs to regenerate. Who knows how many more places there are out there, but it's an issue I think is pretty much 'problem solved' :)
As a side note, I'd like to address the people who claim the extra faces who appeared during the mind battle between the fourth Doctor and Morbius in "The Brain of Morbius" are representing other regenerations of The Doctor. In "Mawdryn Undead" the fifth Doctor spells it out point blank. Her will not help Mawdryn and his friends because there are eight of them, and he has regenerated four times so it would be the end of him as a Timelord because he only has eight regenerations left. So you can choose to believe the ambiguous appearance of unexplained faces or the facts that are spelled out and spoken by The Doctor himself.