I don't think anyone knows yet, though I've seen plenty of theories about it. For me, in the manga, I have difficulty telling that Mikasa has gotten older in his dream. The only difference I noticed was her clothing. If it wasn't for Eren commenting on it, I would have never noticed. But here someone has given their opinion on the matter.
Remember that Isayama said that when he started Attack on Titan he had already planned how it was going to end, and that it was going to be a tragic finale where everyone dies; but he changed his mind due to the success of the series and now the finale will be different (and obviously we don't know how it will be). So it's very likely that in that scene Eren was having a premonitory dream where he was watching that finale and an older Mikasa saying goodbye to him before dying, though it is probably going to be changed in order to fit with the new finale.
About the title, I have always thought that the series will have an epilogue at the end where we will see the descendants of Eren from 2000 years in the future, either remembering the victory or the defeat of humanity, depending on what will happen.
I though it was moments before Eren triggerd the coordinate
I think it was Ymir. Chapter 122 Titled "From you, 2000 years ago", a play on the first chapter's title of "To you, 2000 years from now"
Eren to Ymir; "Is it you who led me all the way here?" "You have been waiting all this time... Since 2000 years ago, for someone."
The title similarities and this quoted text makes me over 90% sure.
Edit, I answered a different question.
Eren was crying because he just finished dreaming all of the events that we have been reading. Just like how Eren was able to convince his father to eat and kill the Fritz family through the power of the Assault titan, Eren sees these memories. And he cries.
This is how Eren Kruger, when saving Eren's father, was able to speak Eren and Mikasa's names, even though they were not even concieved of yet. He was able to see into the memories of the future holders of the Assault Titan.
TL:DR, Eren just dreamed the memories he would one day obtain when he gets the Assault titan, the only titan whose memories can see past AND future. (That we know of)