The Partial Regeneration
The Doctor regenerates, but into David Tennant. Surprise! The reason is either that it was a glancing blow, or because the Earth is one second out of sync with reality. The Doctor fights the Daleks, gets to Davros' control room and manages to hit the time-out-of-sync button (tm), thanks to Donna sacrificing herself, which puts the Earth back where it came from. This resets all the timelines. Rose is still stuck in a parallel universe. Torchwood survive, unfortunately.
Likelihood: High
Opinion: Annoying
The Doctor regenerates, but into David Tennant. Surprise! The reason is either that it was a glancing blow, or because the Earth is one second out of sync with reality. The Doctor fights the Daleks, gets to Davros' control room and manages to hit the time-out-of-sync button (tm), thanks to Donna sacrificing herself, which puts the Earth back where it came from. This resets all the timelines. Rose is still stuck in a parallel universe. Torchwood survive, unfortunately.
Likelihood: High
Opinion: Annoying
My theory is along these lines, but I think the hand-in-jar is going to regenerate as well, so we get *two* David Tennants. Then one of them can live happily ever after with Rose.