Re: 800th Episode - 2014 Christmas Special (Rumour's/Poss. Spoilers)
I don't think Capaldi will leave after just one season; it wouldn't be very fannish, and nor would it really be fair to existing viewers. No real idea what his Doctor will be like, but I think he'll want to give younger viewers fond memories of a "Capaldi era" rather than just being an interim Doctor who essentially lasts just three months. I'm thinking three, maybe four seasons.
"Fantasy is an exercise bicycle for the mind. It might not take you anywhere, but it tones up the muscles that can." - Terry Pratchett.