Journey's End is almost (almost) worse than Doomsday. Not even because he's lost Rose again, but because he got everyone back, everyone he cared about, everyone who helped him survive, and lost them all. It's no wonder regeneration wasn't long in coming after this.
Billie just said this at a convention; no one has actually asked her to do this. But wouldn't it be so amazing?! You'd get to watch the Doctor learn to cope with living as a normal human... without a TARDIS or sonic screwdriver. -- More importantly, we would get to gaze at David Tennant again, not that we ever actually stopped.