WARNING: The following contains spoilers for the Attack on Titan anime and manga.
Hajime Isayama's Attack on Titan has a legacy almost as complicated as the series' own history. Even more complex still is the series' relationship to time and place, which become more and more distorted as protagonist-turned-antagonist Eren Jeager amassed ever more incredible Titan Shifter powers, including being able to see both forward and backward in time, to a certain extent. With this knowledge, fans have been able to reassess even the most foundational elements of Attack on Titan, such as the meaning of the series' first manga chapter and anime episode's shared title.
"To You, 2,000 Years From Now" was a strange name when both the first chapter and adapted episode were released (it's slightly altered for the anime), and it remains strange throughout much of the rest of Attack on Titan. 2,000 years from when? And who is the "you?" Before the manga concluded earlier this year, time-travel theories were abundant in the series' fandom, and for good reason. That title alone implies some kind of future sight, which, before Eren unlocking the full powers of two of his Titans confirmed this to be the case, were compounded by small yet significant clues: Ymir's discovery of tinned food in a foreign language at Utgard Castle insinuated the existence of an advanced culture in the past or present, while Eren Kruger's foreknowledge of Armin Alert and Mikasa Ackerman's names years before they were born teased the Attack Titan's true, time-manipulating power.
Even before Attack on Titan ended, the real meaning of this title, specifically, could be found scattered in other contextual clues. In Chapter #55 of the manga (Episodes 39 - 40 of the anime), Commanding Chief Dot Pixis describes the Fritz bloodline as being "unbroken for 2,000 years." This refers to the fact that two centuries ago, Ymir Fritz became the 'mother of all Titans,' being the first human to ever gain the power of the giants from what's later revealed to be an ancient worm-like creature. Her last living descendants during the time of Attack on Titan's story end up being Historia Reiss and Zeke, but, more importantly, the "unbroken" part could also refer to Ymir's Founding Titan, which, until it was stolen from the royal family by Eren and Zeke's father, Grisha, hadn't left her bloodline since her death.
This makes "To You, 2,000 Years From Now" sound like it comes from Ymir Fritz's perspective. It's not until Chapter #86 (Episodes 56 and 57 of the anime), however, that Ymir is revealed to be the Founder of the Titans, which more or less confirms that point of view, and not until Chapter #122 (which has yet to be adapted in the anime) that her full origin as the enslaved wife of King Karl Fritz, cursed to carry on his will from beyond the grave, is fully explained. That chapter also flips the first's title: "From You, 2,000 Years Ago," as it looks back into the past rather than forward into the future (or, the present, from our perspective.)
This explains what 2,000 means and from whom the statement originates, but what about the all-important "you" part? For that, we can make an educated guess based on the final two chapters of Attack on Titan. Isayama keeps readers guessing right up until the last few pages of his tumultuous tale of inter-generational and racial violence as to how the chips will fall, and fall they do. Thanks to Eren being given access to the Founding Titan's full power -- and his mortal body's death leaving him a mere consciousness suspended between life and death -- he learns that Ymir is similarly trapped in a metaphysical hell of her own making; her love for an abusive husband keeping both her and her Titan progeny from disappearing from the world.
Ymir desperately needed someone to do what she didn't have the strength to. That person turns out not to be Eren, once presumed to be her liberator, but Mikasa, who kills what's left of her beloved adopted brother -- also the object of her own romantic affections -- inside the Founding Titan to end both his reign of terror and the Titan race. As she does, Eren finally finds peace, having martyred himself as a common enemy for the world to unite against, while Ymir's reaction is a simple smile, indication enough that this is what she wanted, all along. Therefore, the "you" in "To You, 2,000 Years From Now" is most likely Mikasa. Two women, two centuries apart, trapped by their devotion to villainous men, finally free.
Though there isn't any time travel in Attack on Titan, the cyclical truth of the story comes into full focus during Eren's final moments, as well as Ymir's: The series starts with him waking up from a weird dream underneath a tree on a hill, in which Mikasa bids him good-bye, and ends in the exact same way. Later, Eren is buried under that same tree. Like Ymir, Eren knew -- without really knowing -- how his life would end all along; tragedies neither of them could ever escape.