This is speculation for the fun of it while I’m bored in work on a Monday morning, but this is my theory – that Peter Capaldi will be playing The Valeyard rather than a “standard” regeneration.
The last seasons saw at least one reference to The Valeyard appearing. Within the mythology of the show, The Valeyard is a darker version of the Doctor that appears between the 12th and 13th regenerations, a penultimate regeneration. The reveal of John Hurt as the 10th Doctor means that the current Doctor is the 12th and the next regeneration would be the 13th and the last.
Capaldi is quite a high profile actor and as pointed out has a lot of other work. I can’t see him agreeing to a long term contract so I see him doing a single season, maybe as “the dark Doctor” before being replaced with a younger, unknown actor who is happy to play the same role for the next few years.
Again, just fun speculation, but could play out like this:
Whatever happens in the 50th Anniversary, whatever caused the shame and/or guilt that made the Doctor hide away the memory of 10 (has to be related to the Time War) to be hidden away causes the next regeneration to go “wrong” and create a dark and brooding Valeyard-Doctor who is in constant struggle with his evil side for a season. Perhaps he is pursed by a Watcher who is the actual 13th regeneration?
The 2014 Christmas special has the actual 13th Doctor appear at the end.