Spoilers for absolutely everything, obviously.

The giant tree looks identical to the one seen during Ymir’s backstory. It is same tree that the Hallucigenia (founding titan, source of all life itself) was underneath two thousand years ago. The modern tree is the one where Eren used to sit as a child, it is also his final resting place. Now because he was buried there and because the tree grew incredibly tall, like unnaturally means that the founding titan, Hallucigenia or well life lives on! What is presumably Mikasa’s grandchild or even great grandchild stumbles upon this tree and if the child were to walk in, the cycle would repeat itself. A new Ymir is born. So the tree represents the cycle of revenge and bloodshed that seemingly was broken piecing itself back together.