Perhaps it could be an old acquaintance from her past who re - enters her life. Like an old high school or college sweetheart that she had a falling out with but then they had a brief reunion and that's how she got pregnant.
One idea could be that Sue discovers she's pregnant and has to deal with the reactions from the glee club members. She might initially try to hide it as she doesn't want to show any vulnerability. Then as the pregnancy progresses, she starts to soften a bit and the students end up being surprisingly supportive.
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 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.
Maybe it's a new character introduced just for this plot twist. This would add an element of mystery and new drama to the story.
It could be Ron. Since they have a relationship in the original story, it's a very likely option. Their relationship could be further developed as they face the new chapter of becoming parents together.
Perhaps it's a human boy who accidentally wandered into the fairy world. Tinkerbell, being curious, interacted with him, and in a magical turn of events, she became pregnant. This would create a lot of drama as the fairy world and the human world don't usually mix in such an intimate way, and there would be a struggle to figure out how to handle this situation.
It could be her long - time boyfriend. They might have been planning for a family in the future, but the pregnancy comes as a bit of a surprise earlier than expected.