I agree with this theory more and more for various reasons but in my opinion (at least regarding the TV series), the Three Eyed Raven we saw in the show was Bran in the future. Here are the clues I was able to gather:
1) The Three Eyed Raven told Bran: "The time has come...for you to become me" 2) The Three Eyed Raven argued he was Bloodraven but in the TV show we can clearly see that he has got two perfectly working eyes, whereas Bloodraven lost one. In the books, the Three Eyed Raven has only one eye, but Bran can still lose one on the way. I think the Three Eyed Raven tells he is Bloodraven to trick Bran and hide the fact that he is him: Bran himself was not ready to learn about it and would have gone crazy if it had been revealed he was being mentored by a futuristic version of himself. 3) Which man with two good legs would tied and "chained" himself to a tree? Bran can't walk and will never walk again so it makes sense if he ends his life tied to a weirwood tree. In the books Jojen Reed dreams of a winged wolf bound to the earth with grey stone chains. It's a good metaphorical for Bran chained to the earth through the weirwood roots, isn't it? 4) We know they have similar powers but the Three Eyed Raven's are more powerful and mastered: it makes sense if Bran has the time to master the skills until he gets tied to the tree. 5) He told Bran: "You will never walk again but you will learn how to fly". The Three Eyed Raven wargs into a raven doesn't he? 6) Jojen Reed tells Bran: "You can't kill it [...] because the raven is you". 7) The Children of the Forest consider the Three Eyed Raven as the last greenseer but we know that Bran is a greenseer (he can warg into weirwood trees) so it would make sense if Bran is the last greenseer. 8) The Three Eyed Raven tells Bran: "We watch, we listen, and we remember. The past is already written. The ink is dry." Maybe its because he knows already what is going to happen. 9) The Three Eyed Raven knows an awfull lot about Bran and his companions. It makes sense if he has lived this life already. Moreover he doesn't tell us much about the rest of the world but his knowlegde is focused on Bran. 10) If Bran's mark can lead the Others through the Wall's spells then Bran is doomed to stay beyond the Wall for the rest of his life. It would then make sense why the Three Eyed Raven stayed beyond the wall.
My theory implies that time works in circles or loops. I've also found one point that goes against this theory: if the Three Eyed Raven was a future Bran then he would have been already marked by the Others and the spells of the cave wouldn't have worked in the previous episodes. Or maybe Bran will find a way to erase the mark.
What do you think?
Sorry for my English, I'm not a native speaker.