I enjoyed TDZ's ideas about the forthcoming regeneration, so here is another.
I'm not sure if it is still prevalent but there was a time when the reason given for the first Doctor's regeneration was that he hat stayed in his body for too long, he liked it, did not want to let go and would have died if not for the TARDIS giving him a nudge, accepting that change was necessary.
In many ways it seems that Peter Capaldi has reached a similar point. He has lasted thousands of years, changing enough times that his identity has been subsumed by all the 'other' Doctor's he has been. He just wants to be himself. (It could be he is rather attached to the Capaldi Doctor, or he is yearning back to his first incarnation).
There is a similarity to the mind set of the two Doctor's and I have a feeling it is going to be a two way thing.
Hartnell, lost in the snow is from the Tenth Planet, resisting change. Capaldi's Doctor is going to convince the First to change, that they have seen so much in the intervening years, done so much good, met so many people that it would be a crime not to live it. Convinced to change the first Doctor will somehow return the favour, suggesting that if the modern Doctor has got to the stage where he feels lost and just wants to be him, perhaps the time is right to change completely. A different perspective completely.
I'd be quite happy with something like that.