Is the Rassilon that we saw the real one? The one locked up in the tower. I thought I read somewhere that someone didn't think that the EOT Rassilon was the real Rassilon, it was more of a title that was passed.
However, I do believe it is, and I have a reason behind it. I've been reading Celtic Myth and Magick by Edain McCoy. When it talked about certain heroes and gods, it would be mentioned that they're Sleeping Gods, awaiting to be awoken because the country needs them at that point. My thought is that is the condition of Rassilon, he was a sleeping god(?), waiting to be awoken when the planet was in real peril.
I also think that Rassilon took the remaining life out of the people that claimed immortality, regenerated with one of Borusa's and that's why he was screaming I WILL NEVER DIE.