If there is a missing Doctor (between the 8th and 9th?) then that would make Tennant's death the Fall of the Eleventh, right?
So, perhaps the place the Doctor is not supposed to go is Tennant's regeneration.
The main problem with that idea is that Tennant will reprise his role in the anniversary episode, not the season finale in which you would expect him to appear if the theory is true.
"The trouble with fighting for human freedom is that one spends most of one's time defending scoundrels. For it is against scoundrels that oppressive laws are first aimed, and oppression must be stopped at the beginning if it is to be stopped at all."
-H. L. Mencken