It could be her long - time love interest. In many fanfics, the most obvious choice would be the character she has the strongest romantic connection with in the original series.
Another possibility is that it's a character that was a bit of a side character in the original but has a hidden connection or a growing relationship with aphmau in the fanfic, which leads to the pregnancy.
Maybe it's a new character introduced specifically for this fanfic. This new character could bring a whole new dynamic to the story, with a different personality and backstory compared to the existing characters.
Or, in a more creative take, it could be a new character introduced just for this fanfic. This new character could bring a whole new set of dynamics and storylines, like a mysterious stranger who has some special connection to Clary.
Most likely, Percy could be the father. They have a strong relationship in the original series, so it would be a natural choice. Their chemistry and shared adventures could be the basis for a story where they start a family together.
Maybe it's a completely new character introduced in the fanfiction. This new character could have some special powers or a unique connection to Naruto that results in the pregnancy.
It could be Beast Boy. Their relationship in the original series has some chemistry, and in the fanfiction, it would be an interesting twist if he were the father. Maybe their relationship takes a new turn when Raven discovers she's pregnant.
Perhaps it's a new kind of monster that was created through a combination of human technology and some ancient, unknown force. This new monster mated with Godzilla in some unexpected encounter, and now she's pregnant. There could be a whole backstory about how this new monster came to be and how it interacted with Godzilla.
It could be Rick. They have a strong bond and connection in the series, so it's a possibility that he could be the father in the fanfiction.
If we follow the more traditional route, Mako could be the father. They had a relationship in the show, and it would be a natural progression for them to start a family. Their relationship had its ups and downs, and the pregnancy could be a new chapter in their story together.
One possible storyline could be about the challenges she faces during pregnancy, like dealing with morning sickness and hormonal changes. Another could be how her friends and family support her throughout the pregnancy journey. And perhaps there could be a storyline about her preparing for the new arrival, such as setting up the nursery.
While Peeta is the obvious choice, some fanfictions might explore the idea of Gale being the father. This could add a whole new layer of drama and conflict. Maybe there was a moment of passion between Katniss and Gale before she fully committed to Peeta, and this led to her pregnancy. However, this would be a more controversial take as it goes against the main narrative of Katniss's relationship with Peeta in the original series.
It could also be a new character introduced in the fanfiction. Maybe a powerful demigod from another camp who Annabeth meets during a diplomatic mission. This would create a whole new set of drama and conflict, as Annabeth would have to deal with her feelings for Percy and this new person, as well as the implications of having a child with someone other than Percy.