It could also be someone new, a character created specifically for the fanfic. Maybe a new hero who has come to their world and Annabeth has fallen in love with him. This would add an element of novelty and new relationships to explore in the story.
Another possibility is that it could be a character from her past that she re - connects with. For example, an old friend or acquaintance who she had some unresolved feelings for. This could lead to a story full of twists and turns as she deals with her past and present relationships while being pregnant.
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.
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.
Another possibility is that it could be a god in disguise. Given the complex relationships between demigods and gods in the Percy Jackson universe, a god could have fallen in love with Annabeth and fathered her child. This would bring all sorts of divine drama and complications to the fanfic.
There's a chance it could be a new character introduced in the fanfiction. Maybe someone who has a mysterious past and enters Annabeth's life briefly but leaves a big impact, resulting in her pregnancy.
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.
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.
It could also be someone new, a character created specifically for the fanfiction. Maybe a powerful new demigod who catches Annabeth's eye, and their relationship leads to the pregnancy.
The fanfic could also take a more complex turn. Annabeth gets pregnant at a time when there are threats to the world. She has to deal with the fact that she's bringing a new life into a potentially dangerous situation. There could be a prophecy related to her unborn child, which adds more drama. Her pregnancy might also be a sign of hope for the future of their world, as the child could be a great hero in the making.
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.