You loyal SnK theorists have probably come across this theory. But for the new ones, according to this theory Eren's a time traveler (Not physically but consciously/mentally) and he's sent to prevent extinction of humans(Kinda like X-Men Days Of Future Past). Starting point of this theory is the very first chapter of the manga and first episode of anime. Both named "To you, 2000 years from now". Considering all the names of episodes/chapters are important by themselves, it's an odd name. What does that mean? In the theory, it's a message for Eren from a dystopian future (yes, even more dystopian than current world) where humanity is about to extinct and key moment that leads to this catastrophe is the time when SnK events occures.
Another evidence that support this theory is the dream of Eren. In manga, it's Mikasa with short hair that says "See you later, Eren." While in the anime, it's a bit longer. Eren sees couple of visions. While many of it is up to debate, 2 of them are pretty clear. One, titans' invading inside of the walls. Two, iconic scene of Eren's mother's getting eaten by Smiling Titan. You can say "Hey, what's in the anime does not matter." but let me remind you, mangaka Hajime Isayama plays an active role in anime. He even does things he couldn't do in manga (Like Annie's reaction to reveal of her identity). Either way, one can not deny that, Eren sees the future in his dream.
Now the theory is explained, let's talk about its flaws. First, if it takes place in somehow 2000 years after from now, there's no possibility of either Mikasa's or Eren's staying alive. As far as we know, both titan shifters and Ackermans do age. Second, name of the episode might be a reference to the real world. SnK world might take places 2000 years after our world. And third, this may be a cancelled scenario. As we know, Hajime was planning on killing either all or most of the characters in the end of the manga but due to the popularity of anime, he changed it. So, this dream or name of episode/chapter might be outdated.
You can read longer version of this theory from here .
What do you guys think about this theory?