Maybe Dan. Since Tsunade still had strong feelings for Dan even after his death, a fanfiction could explore a supernatural or alternate - reality scenario where he is the father.
It could be Jiraiya. Given their long - standing relationship and his often - flirtatious nature with Tsunade, it's a possibility.
It could be one of her close friends from the guild. Maybe Jellal, considering their complex relationship in the original story. There's already a lot of history between them, and this could be a new development in their relationship.
It could be a long - time boyfriend that we haven't known much about in the original story. Maybe he's a character who was in the background but now becomes central to the plot.
One possible storyline could be about how Tsunade deals with the unexpected pregnancy in the context of her leadership role. She might have to balance her duties and the changes in her personal life.
It could be her existing partner from the original story. For example, if it's a story where she's in a relationship with Edward, then he would be the likely father.
There's also a chance that it could be a god in disguise. Gods have been known to have relationships with mortals and demigods. So perhaps a god took on a human form and had a relationship with Thalia, resulting in the pregnancy.
It could be Jiraiya. They have a long - standing relationship. Jiraiya has always been around Tsunade, and they have a deep connection. His carefree yet passionate nature might be what Tsunade needs in a partner to start a family with.
His mother Tsunade could be one of his first and most important allies. She would surely use her influence and skills to protect and guide him. Shizune could also be an ally, as she has been with Tsunade for a long time and would likely support the OC. Additionally, Naruto could be an ally, as he is known for befriending and helping out those in need in the ninja world.
It could be his long - time partner. They might have had a very loving relationship that led to this situation.
Perhaps it's a mysterious stranger she met at a party. They shared a brief moment together, and now she's pregnant. Or it could be an old acquaintance who has always had feelings for her and took advantage of a vulnerable moment.
Maybe it's a new character introduced specifically for the fanfiction. This new character could be from a different faction or group, and their relationship with Makima could be full of intrigue and mystery. The pregnancy could be a result of some sort of strategic alliance or a passionate encounter between them.