Jumping in here to say that it's an open-ending for a reason. The story is about human nature and the cycle of conflicts. The world in AoT has always been grim and brutal but never without hope. The ending suggests that history may repeat itself, but there is also reason for us to hope for a different future because the child at the end is very different from Ymir. While Ymir ran for her life, pursued by hounds, the child is on an adventure, carefree and happy with his dog companion. If Titan power was created by Ymir's repressed desires for freedom and hatred for the world, then the power this child creates, if one was created at all, will be very different :)
I wouldn't say Eren "sacrificed" himself because that implies nobility on Eren's part. Eren wanted freedom and was willing to sacrifice the world for it. He started the rumbling, knowing full well that his plan to exterminate the world would eventually fail.
Eren's objective was freedom for himself, for Paradis, and the safety of his friends. He never meant to achieve peace, because he knew full well that human nature would always entail conflicts and wars. Even if he did succeed in exterminating the world, Paradis would probably start fighting among themselves, as did the Eldian Empire after conquering the world.
In the end, Eren did get what he wanted, except freedom for himself, because ironically he was a slave to freedom itself.
Is history going to repeat itself?? The ending scares me. What did the child see in the tree? Someone explain please.