Eh, it wasn't exactly all my theory, there were a decent bit of us on Reddit speculating and trying to tie clues together, haha.
However-- so glad the theory was correct... IT WAS DONE SO FANTASTICALLY. Literally the best episode this season and one of the best episodes in the series overall, if not THE best. Like, the feeling of the episode was so foreboding, the sacrifices were absolutely crushing and the how everything tied together is just amazing.
SPOILERS below if you haven't seen the episode...
John Hurt. Time for some random theorizing. Here's my ideas:
-The most obvious solution is that he's the Doctor from the Time War as it is mentioned that what he did was horrid and despicable, and that he did it "without choice...in the name of peace and sanity..." ...but not in the name of the Doctor." (clever title-reference, Moffat!). However, before this, everyone assumed that the Eighth Doctor had been the one to deal with the Time War, and we'd never really seen the Doctor react this way. Saying what was done was not done "in the name of Doctor" perhaps contradicts what had been said before (things said by Ten and Nine, as those were usually from their perspective ("I killed my entire race..." "The Doctor has the Moment...")) so perhaps...
-It's an entirely different situation. Something so "horrible" that actually brought about peace and sanity sounds about par for the Time War, but perhaps it's something Moffat has engineered as an entirely new plot thread.
-It's the Valeyard. The Valeyard was specifically mentioned in this episode, which lends credence, maybe, but we all know Moffat and how he likes to lead us in the wrong direction. For those not in the know, the Valeyard was in the 6th Doctor's term, in the Trial of a Time Lord series, and he was referred to as "an incarnation formed sometime between The Doctor's 12th and 13th regeneration..." He was a dark, evil manifestation of the Doctor, something we've seen referenced with the Dream Lord, so perhaps this is his return.
-Perhaps he's not the Doctor at all, and is a Doctor impostor.
HOWEVER, theories one and three could potentially tie into each other. During the Time War, time lords from throughout all time streams were pulled from wherever they were in order to fight in the war. It could be entirely plausible that the Valeyard was pulled into the Time War as well, and his bloodthirsty, dark nature was perfect for what happened, and absolutely needed to be the one who killed all of the Time Lords and the Daleks. The Doctor could have witnessed this, and recognized that what went down was essentially still by his own hands, as the Valeyard is a subsidiary of the Doctor, thus referring to himself when speaking of the Time War and the destruction he caused.
Other speculation: River's whole "we had a connection, and she's dead, so how am I still here? SPOILERS LOL" will definitely be a plot thread. I'm guessing it was a callback to Hide, as the psychic had a connection with a future relative time traveler, so does that mean Clara and River are related? Perhaps. Perhaps not. Maybe they are related in a way that isn't blood, but something much stronger.
More thoughts:
-The ending with the leaf was brilliant and absolutely a fantastic way to tie that all together. I love you, Steven Moffat.
- River's goodbye was essentially the saddest thing ever, especially with the whole "goodbye, sweetie..." that totally broke my heart. I suppose we won't be seeing Alex Kingston from now on....
-Clara and the Doctor are starting to seem even more romantic now... his whole "you're my impossible girl" embrace at the end seemed like a moment that every fangirl will proclaim as the pinnacle beginning of a Doctor/Clara relationship, lmao. It was really sad seeing River's face realizing who the Doctor's new companion was. However, it was a bit odd-- at the beginning, it seems as though River does not recognize Clara, but her knowing something about her and Clara's connection would hint that she has met Clara before.
-The footage of all eleven Doctors was so lovely. Having Clara be in all that old footage (and the scene with the first Doctor choosing a TARDIS HOLY CRAP) was absolutely outstanding and definitely a huge throwback for classic fans.
Overall, probably the best finale in the history of Doctor Who. Series 7 just shot up the list to either my second favorite series, or potentially my favorite entirely. Brilliant work.