There's been some discussion about whether Donna might turn out to be a Time Lord (and yes, on a third watching/listening I agree there could be a second heartbeat while Donna is in the Shadow Proclamation), specifically Romana - with that ring perhaps being the female ornamental equivalent to the watch used by the Doctor and the Master in the previous season. One thing kharin likes to point out with this theory is the symmetry between Richard Dawkins appearing in The Stolen Earth and a potential appearance of Lalla Ward somehow in Journey's End.
But what really triggered another thought is the Doctor's reference to Donna's Grandfather at the end of Turn Left. There's a lot of pointers in The Stolen Earth to The Dalek Invasion of Earth. I wonder, therefore, whether Donna could be Susan. Of course, this relies on classic Who knowledge, and is therefore almost certainly beyond the scope of RTD's writing. Therefore, the backup to this idea is that Donna will turn out to be Jenny. Which I find much less satisfying, but would tie up that loose end from earlier in the season (assuming Jenny's future doesn't lie in a spin-off series to replace Torchwood).
None of which has the slightest bearing on the fate of the Doctor or Harriet Jones. But I have to take my hat off to RTD for engineering so much effort in our speculation.
And what of the Brigadier? Well, he must be so long retired by now, he should be bee-keeping in the true Conan Doyle tradition. But if the bees have gone ...