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.
It could also be another demigod. Maybe someone she met during a new adventure or quest. This would add an interesting twist to the story as they'd have to figure out their relationship while also dealing with the pregnancy.
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.
It might be an old enemy who somehow managed to get her in this situation as part of a devious plan. This would add a lot of drama and conflict to the story.
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.
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.
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.
It could be a long - lost love from Mavis's past. Maybe someone she thought she'd never see again but who re - enters her life right before she discovers she's pregnant.
It could be another powerful magical being in her realm. Maybe a long - time ally or a mysterious figure that has been in the background of the story's world. This would add an element of mystery and intrigue to the fanfiction.
In some fanfictions, the father could be a human character. This would add an interesting dynamic as it would involve dealing with the differences between a human and a Digimon in a relationship and during pregnancy. It could also lead to a story about acceptance and dealing with the unconventional nature of such a union.
It could be one of her close friends or classmates in 90210. Maybe someone she had a brief fling with during a party or a school event.