The rumour I heard was that Tennant would leave after series 4, only having signed up for three series of the show (three being the average life of a Dr).
The other rumour, which perhaps relates to this, is that Russell T Davis would leave after the fifth series. It would then be passed on to someone else. I think this sounds the most logical, as Dr Who is fast pretty much one of the most popular shows the Beeb have at the moment.
Obviously, this gap year is rather suspicions. Rumour also has it though, that the next Dr has already been selected.
So who would you put up as the next Dr? I've been told, that the Beeb want a younger, up-and-coming actor, under 40, who will make women watch. I think that rules out anyone I'd pick... Personally, I'd like Eric Idle. Or maybe James Nesbitt, he was good in Jekyll. Whoever it is, they have to be able to pull of quirk and comedic, yet play it as if it was 'real'. What we don't want is another Berty Bassett monster. No thank you, Mr McCoy...