The Attack on Titan world is almost unrecognizable from when the anime just began. The lore is ever-growing and the plot has expanded to unimaginable lengths. The universe created by Hajime Isayama has spawned countless theories about the fate of characters and events in the storyline - backed up by things found in the manga and anime.
One theory that is quite interesting surrounds the father of Historia's unborn baby. It is suspected that the father is not the Farmer as Historia claims, but rather Eren Yeager. While this is ultimately just a theory, there are details to suggest that this may actually be true.
Eren & Historia's Relationship
Eren and Historia became unlikely friends throughout the Attack on Titan series. The two surprisingly share many similarities in their lives. They understand each other on levels even Mikasa can't fathom. Both of them have so many unanswered questions about their families, roles and what that means for their futures.
Eren never bought her Krista Lenz persona. Krista was sweet, innocent, kind, an angel. He said she came across as fake, unnatural and creepy. Her "nice girl" act was not genuine to him. Now that she revealed her true self, she does not have to hide behind pretenses anymore, which Eren respects a lot more. Her brutal honesty makes her normal, which Eren prefers. Historia blossoms into a stronger person, which Eren notices and envies. Historia envies Eren, too, for knowing exactly what he wants.
Their ordeal in the crystal cavern cemented their friendship. Eren was at his lowest, begging Historia to eat him and end his suffering as a carrier of the Attack Titan and save the world. Historia lashes out that she does not care about humanity and that Titans should ravage the world anyway, which makes her humanity's worst enemy and the most despicable girl in the world. However, she saves Eren because he is her friend. She knows what it's like to feel unwanted, useless and a disappointment; so she cannot bear to hear Eren talk about himself this way. She is not claiming to be a good girl, but she will rescue anyone that felt what she did all her life.
Why Does Historia Have to Bear Children?
At one point, Zeke Yeager devised a plan to protect Paradis. He rekindled an old alliance with the foreign nation of Hizuru. He wanted to unleash a fraction of the Rumbling to show the rest of the world its destructive capabilities. Hizuru would then lend their military until the Rumbling is no longer a threat.
Hizuru's ambassador, Kiyomi Azamabito, held that the power of the Founding Titan and a Titan of royal blood must be maintained. Therefore, Zeke must bestow the Beast Titan to someone of royal blood who would need to produce as many heirs as possible throughout their thirteen-year lifespan. As the only known member of the royal family, this duty falls to Historia. For the sake of humanity and saving her people, Historia agrees to comply with this plan.
The Farmer
In episode 10 of the anime, it is revealed that Historia is now pregnant, and quite far along at that. An unnamed man referred to as the Farmer was taking care of her, the supposed father of the unborn. The story is that the Farmer is a boy from Historia's childhood. When they were both children, he would throw rocks at her because she wouldn't leave the farm, perhaps as a way to get her attention. Out of guilt for treating her this way, he volunteered at Historia's orphanage doing menial labor; this caught Historia's attention.
Eren's Place in This
So where exactly does Eren fit in with this debacle? In season 4, he was the one who informed Historia of the plan for her to eat Zeke and sire as many heirs as possible. While she is content with this plan, Eren is not. He does not want a cycle to exist on Paradis where children exist to eat their parents merely to inherit the powers of Titans. He wants to end this vicious cycle of revenge "born out of hate".
Additionally, he wants to destroy all Paradis' enemies, which Historia is dead set against. She does not want innocent people to die from Eren's dire decision. After all, Eren's own mother died as a casualty when Bertholdt's Colossal Titan broke the wall at Shinganshina. Eren offers to alter Historia's memories witht he power of the Founder if this plan is too much for her to bear.
The Theory That Eren is the Real Father
Historia was pro Zeke's plan to bear children since the start, because she felt that would save her people. Practically speaking, she probably would want to have a child with someone she trusts and knows. No other male character is shown to be as close to Historia as Eren is, therefore it is not a stretch to imagine that she asked him to be the father.
Eren's dream was to create a world where people have freedom. As a royal descendant, Historia and her descendants are important to this. Historia fell pregnant before Eren left Marley and his home was not far from her. Floch was entrusted to guard Historia. By telling Floch and Historia his plans, Eren was able to spread word of his intentions. By impregnating Historia, he would protect her and eradicate the Curse of Ymir. These events took place approximately ten months ago, the pregnancy period for Japanese women. Additionally, Historia lied about how far along she was. She gave birth three days after Levis visited her, where he figured that she had a few months till her due date.
Why would she lie about this? It is possible that she knew of a bigger ploy, and Eren is the only character thus far with a hidden greater plan. This implies that she is complicit in mothering Eren's child. She also cannot disclose the real father of her baby, because Eren was planning a coup d'etat. The Farmer is a possible stand-in for the father. He is such a random character to be introduced at such a pivotal event in the story. We don't know anything about him. From a story-telling perspective, it seems odd that the father of Historia's child will be someone so unknown. Historia also seems very unhappy when she approaches him.
In 2018, Isayama revealed the final panel of manga which featured a man with neck-length hair holding a baby, saying "you are now free". It is possible that this man is Eren, because he is the main protagonist and has long hair. Given that Historia is the only known pregnant character thus far, it is plausible that this is her child. Historia also gives birth when the Rumbling begins, which is too much of a coincidence. The theory that suggests that Eren is the father ties together the death of the old world, and the birth of a new one. A new generation that are free is what Eren wanted all along, after all.
Sources: K-AMINO; Anime Balls Deep