Disclaimer: I don't own Snk
There are manga spoilers, read at your own risk
Eren
The price of freedom was a high one. It had costed him almost everything and almost left him with nothing. He sat in a rocking chair holding his child, his face emotionless. He looked off into the distance. The sun was setting behind the mountains. It was almost dark. It was time for him to go inside. Ymir was sleeping soundly, her small head resting on his shoulder. The child and his wife were the last remnant of his humanity. They kept him tethered to this world. They were the reason he wasn't dead, why he didn't kill himself. They were the reason for him to keep moving forward despite everything he had done.
His child was only a couple of weeks old. Historia had given birth a few months after he came back. He had kept his promise. When the child was born, he cried. Every horrible thing he had done, all the sacrifices that were made... were for this moment. He had picked up his child and pulled her into a hug.
"You are free." He had said in a broken voice as he cried his heart out. It was true in every sense of the word.
All the hate outside Paradis was gone, forever buried under gigantic footprints. His colossal Titans had scarred the world but life began to regrow. Trees were coming back to life, buildings were becoming buried under sediment. Winter was over, spring had arrived. A new dawn for humanity was here, more specifically, a new dawn for Paradis. When the Rumbling was finished, he had dismantled all of the Colossal Titans and rather than returning them back into humans, he had finally let them rest in peace.
All of the 9 Titan powers were inside him now. Ymir the Founder had left the realm of paths. Eren had helped her escape, she was freed. No one else would ever have Titan powers again. The curse of Ymir was broken. Eren could now live until he was old and wrinkly. That was his reward, but also his curse. Once he died, the Titan curse would finally have left the world.
When he had came back, almost all of Paradis cheered for him. There were some who were still bitter from losing their families and loved ones under the destruction of the Colossal Titans but they were eventually convinced that he had no time to effectively evacuate everyone before the attack in Shiganshina with the Marleyan warriors. He was greeted as a war hero, it made him feel sick. After everything he had done, he didn't need to be regarded as one. He was a monster, the devil of Paradis.
It took a while but eventually the structure of Paradis was eventually stabilized and he had become the new king of Paradis. The buildings were fixed and his remaining Yeagerists were assigned to become his royal guards. He was respected and revered. His child would be the first royal blood to not have to inherit Titan powers. He only had one thing left to do, the thing he didn't want to do, but it was what he believed was best for his people.
Eren used the power of the founder to wipe the memories of his people. They believed that they were the last of humanity, but this time, it was true. Everyone had their memories wiped except him and Historia. Only they deserve to live with the guilt of knowing the fate of the outside world. That was their punishment until they died. In the case of his people, ignorance was true freedom. They didn't have to worry about how their free lives were only possible because of the destruction of other lives. They didn't have to feel guilt, worry, or sadness. They were free to live their own lives, to choose their own paths. They were free to be happy. There was no one to discriminate against them, no one that would exterminate his people the first chance they got. They would live in paradise.
But it was in human nature to be violent and cruel. Eren knew war would rise again eventually but for now, his people are free.
Everyone but him and Historia are free.