I know this sounds crazy but what if Eren is the father of Historia's child? I have several thoughts on why:
1. Eren will die as the martyr who united the world together for both human and titans, and if you think he is on a genocide route, then go re-read ch131, Eren can see the future, he saw the inevitable future had him leading the Titans, but, importantly, that never meant he couldn't have his allies try and stop him, hence why they were not brainwashed to not stop him. All he knew was it was inevitable he would lead the titans not that he would sit back and accept the fate of wiping 99.9% of everyone off the face of the Earth. Cmon people think, really.
2. While we were told by some high ranking officers, not by Historia, that the father of the child is that of some farm boy who in the past bullied Historia, but also whose face we never saw... cmon I am not buying something so contrived from a series that has not only deceived us multiple times, but also used every little or big scene to guide us to build ups that had surprising and emotional payoffs.
Their would be no meaning if the father was just anyone especially some no name who we didn't even see the face of. The most impactful choice for a big reveal is that the child's father being Erens.
Also, go back to chapter 130, this is the moment when he lays out the plan to Historia, is it not logical to think that they would just agree to have a child together, why is the no name farm boy more plausible here? If it was a titled character who we knew about, then I concede here, but bc it is a no name farm boy, I have the inclination to believe it's Erens.
3. He had a child with her knowing full well what his fate would be and wanted to leave something behind before he died. That might also explain why Historia looked so defeated and crestfallen when she was sitting in that chair pregnant. She knew Eren was never coming back to see their child, bc Eren told her everything and also to not say anything. She did so on the motive to not only protect the island, her people, but also because she is a "bad girl."
4. Furthermore, anybody else find it odd that she is about to give birth at the exact moment this final confrontation is occurring? My take here is when Eren dies it will be in that moment of death his titan (power(s)/being) will be transferred to the next birth citizen (I.E. his child). This idea is backed by how the Marley Country kept mentioning it when they were afraid of one of their titans dying, they said this to not only to instill the importance of not losing a titan in that moment but to foreshadow this exact moment coming up.
This is all just theory crafting, but as far as narrative conclusions go, I can see it happening. I would be satisfied if this is the direction the author is going... but we will see.