"The End of Time" wrapped up the Russell T. Davies era of Doctor Who with a bang, but it left a bunch of unanswered questions behind. Perhaps the solutions are already right there in front of us. Speculation time!
I'll try to avoid any overly obscure questions like, "What happened to that nice doctor fellow Martha was engaged to?" or pedantically fannish quibbles like, "Why does the Doctor keep insisting he's 906 when he was already 953 in 'Time and the Rani'!?" I'll also skip questions like, "Did the Master survive?" because at this point it's pretty clear that the Master always survives, even when he dies. Instead, let's focus on the big mysteries of "The End of Time", in particular ones that the characters themselves note are still unexplained. Let's start with a relatively easy one.
Was that really Rassilon? Like, the Rassilon?
As an uber-geeky old school Doctor Who fan, the most surprising part of "The End of Time" was when the Doctor suddenly addressed Timothy Dalton's President as Rassilon. For those unfamiliar with all of Doctor Who's nearly fifty-year history, Rassilon was known and revered by all Time Lords as the founder of their society. First mentioned in the 1976 story "The Deadly Assassin", Rassilon was credited as the ancient Gallifreyan who harnessed the power of a supernova to make time travel possible. He then became the first Lord President, and ruled for an undisclosed length of time before passing into history.
His biggest role in the classic series was in the twentieth anniversary special "The Five Doctors", in which four of the Doctor's incarnations, a bunch of old companions, and a host of enemies are brought to the Death Zone, a region of Gallifrey used in ancient times for what were essentially massive gladiatorial battles. The whole thing is part of a frankly ludicrous scheme by a rogue Time Lord to create a safe path to the Tomb of Rassilon, which lies at the heart of the Death Zone, so that he can gain Rassilon's secret of immortality. At the very end of the special, we see a projection of the apparently deceased Rassilon, who congratulates the victorious Doctors for playing his game successfully.
So what's he doing, seemingly alive and well in "The End of Time"? There is, of course, a precedent for Time Lords resurrecting fallen members of their race to fight in the Time War. As the Master explained in "The Sound of Drums", this was precisely why the Time Lords brought him back from the dead. If they were desperate enough to gamble on a genocidal lunatic, why not also bring back your first and greatest leader too? Of course, if Rassilon really was immortal - a point "The Five Doctors" dances around - he might just have been resting in his Tomb, awaiting a vast enough crisis to necessitate his return.
Either way, there's a long precedent for his descent into villainy. In "The Five Doctors", the Second Doctor explains to the Brigadier that there were rumors that Rassilon was actually a cruel tyrant, who crushed all who opposed him. The expanded universe has explored this side of Rassilon at great length. The novels Cat's Cradle: Time's Crucible and Lungbarrow, both by Marc Platt (who wrote the classic Doctor Who serial "Ghost Light"), traced Rassilon's rise to power and the often brutal steps he took to consolidate control.
The Big Finish audios have also dealt with Rassilon, casting him a similarly devious light. Voiced by Don Warrington (who played the president of the alternate Britain in "Rise of the Cybermen"), Rassilon lives on in the Matrix, a sort of virtual reality computer system that houses all of Gallifrey's accumulated knowledge. He is depicted as a genocidal figure that has schemed throughout Gallifrey's history to destroy all species that show even the slightest chance of gaining access to time travel. He attempts to use the Eighth Doctor as a weapon against a race known as the Divergents in his final bid to ensure the Time Lords' continued supremacy.
As such, there's every reason to think that that really was the original Rassilon, back in the flesh and supremely villainous. In any event, what's the alternative? That this was just some other Time Lord that happened to also be called Rassilon? I somehow doubt that Rassilon is a common name, even on a planet called Gallifrey. Now let's look at some more difficult questions.
Who was the mysterious woman in white?
This is one is absolutely massive - it's a big enough question that Wilf even interrupts the Doctor's epilogue to ask him about it - but it may already be answered. Pretty much all available evidence points to the fact that Claire Bloom played the Doctor's mother. It was rumored by the British tabloids way back in April, it was sort of confirmed by outgoing executive producer Julie Gardner in the commentary for part two of the "End of Time" (although fellow commentator Russell T. Davies remained far, far more ambiguous on this point), and it's the explanation that's most in keeping with Davies's overall approach to Doctor Who. Case closed. Or is it?