What was the dream Eren was seeing in the first Episode where titans are devouring humans with something stuffs toy and soldiers dead body? Why he was crying? What is the meaning of the first episode name?
Apparaently, this was Isayama's original ending where everyone died. The corpses seen were the survey corp like Levi and the eye was form Eren's own corpse. But since he scrapped that idea, how will he incorperate it in the anime? We still haven't seen a similar scene yet and leaving that scene un connected to the rest of the storyline.
That remains one of the biggest mysteries in the whole story.
Some think that there's a "time loop" of sorts where Eren is trying to prevent the destruction of Paradis, but ends up failing and sends his memories back to when he was young, albiet heavily fragmented to point where Eren only saw those disjointed images in his "dream" until he finds a way to break the cycle.
Others think that because of the "Attack Titan" ability to see the future, Eren saw memories of the future due to the fact that he was "destined" to acquire that power even though he didn't have that power when he had the dream.
What's even more mysterious is that during the end credits of the final episode of Season 3 Part 1, a "glitch" showing a bunch of creepy images occurs and many of those images were actually lifted right from Eren's dream in Episode 1, suggesting a connection between the two events.
It should be noted that Eren's dream in the manga only had a seemingly older Mikasa telling Eren that she'll see him later.
Given that Isayama was heavily involved in the anime production, Eren's dream was probably redone the way it was in order to be better connected to later plotlines he may not have had fully outlined when he first started the manga.