Come back tomorrow for the full version.
Right, I've got it. The Doctor is going to regenerate into someone completely different, either a well-known celebrity (if they want to make it look legit) or someone we already know from the series (if they want to go for comedy value, so Mickey Smith is a very real possibility). Then he's going to die. However, his DNA / Time Lord essence / biodata will somehow be transferred to a human host in the past, and it'll turn out that Donna has been a carrier of Doctorishness all the time. She's called "Noble" because she's a Time Lord, hence the second heart we can almost hear beating in her chest, the ludicrous coincidences, and all that jism about her "sacrifice". In other words, she's going to regenerate back into the Doctor we know, thus qualifying her as his most faithful companion.
And then, and then, and then the Dark-Matter Cybermen are going to turn up with the Zombie Adric and wake up all the Silurians, and... oh, Auntie Em, it was such a strange dream.