Given that the Attack on Titan anime's ending was the same as the one in the manga, as expected, it left fans with a looming mystery.
The final scenes showed fans a glimpse of a mysterious boy and a dog that entered the tree at Eren's burial site. However, the series did not reveal the boy's identity, leaving fans to ponder the same.
The anime's end credits showed an aged Mikasa visiting Eren's grave from time to time, soon after which she passed away. After Mikasa was buried next to Eren, Armin could be seen visiting their graves.
However, as time passed, the land of Paradis became a battlefield, making the place a barren land. As for the tree, it grew incredibly tall.
Attack on Titan: Who is the boy that entered the tree at the end?
There's no direct answer about the boy's identity. It can only be presumed that he was a descendant of one of the series' characters, with the prime suspect being Mikasa.
While the manga did not directly make any such revelations, the illustration of the boy made him look similar to Mikasa. Moreover, he also had a red scarf around his neck. However, one thing is for certain: the boy's presence in the final scenes signified the cyclical nature of Attack on Titan.
After Eren was buried beneath the tree, the end credits showcased how the tree grew incredibly tall, way out of proportion. Thus, it's to be presumed that the Hallucigenia that gave Ymir and Eren their Founding Titan powers somehow either survived or reemerged out of Eren. That might be the reason why the tree grew tall, similar to the size shown in Ymir's backstory.
Even though Eren annihilated 80% of the world's population, Shiganshina developed into a futuristic city. Unfortunately, war again broke out on Paradis Island, leaving the place barren.
Forests soon took over the land, following which the entire landscape looked similar to that of the time when Ymir Fritz entered the tree while fleeing Eldian tormentors.
As for the boy, neither the anime nor the manga has revealed his identity. However, it seems very evident that he's trying to find refuge in a post-apocalyptic war battlefield.
In the process, he found the tree of Eren and Mikasa's burial site, the roots of which have been overgrown, making it look like a cave. The final scene shows the boy and the dog entering the tree.
The only conclusion one can come up with for this ending is that the boy is a representation of the far future, long after Eren’s time. It means that, despite Eren's efforts, Hajime Isayama wanted to depict that the cyclical nature of war was never going to stop.
With the boy entering the tree, it can only be presumed that he was set to receive the powers of the Founding Titan, leading to another two thousand years of torment.