Only Earth has the legend and the lore of the World Tree...which by it's nature and because of it's purpose is immune from paradox. Each branch holds a version of Earth in another universe...as well as that universe. Every Earth can be accessed via the World Tree. It has but one door from the Valley where the Tree grows, but it opens onto any Earth. You can enter the Valley from one Earth and exit it on another, it's all about Intent. Now, suppose the Doctor found his way into the Valley where the World Tree grows...?
And the wonder of it is more that no author I know of other than myself has considered that...and used it. I'd love to see what you could do with a notion like that, since only a Lord of Time could be the gatekeeper at such a place...
It also means that David's 10 or Matt's 12 could retrieve Rose safely.
