Another possibility is that it could be an old flame from Fiona's past, like Jimmy. Even though he had his own issues and disappeared at times, his reappearance in the fanfiction as the potential father could create a lot of tension and bring back old feelings and storylines. It would also make for an interesting exploration of their relationship and how it would develop with a pregnancy involved.
It could be Gus. Since they were married, it would be a likely candidate in a fanfiction. Their relationship had its ups and downs, and if Fiona got pregnant during their time together, it would add a whole new layer of drama.
One possible storyline could be Fiona struggling to balance her pregnancy with her chaotic family life. Maybe she tries to keep it a secret at first to avoid more drama, but then has to face the reality as her body starts to show. Another could be about her relationship with the baby's father, whether he is supportive or not.
Another option could be a new character introduced specifically for the fanfiction. This character could be a powerful pirate or a mysterious figure from Hancock's past. He might have a connection to Hancock's family or her position as a Shichibukai. His presence would add an element of mystery and new dynamics to the story, especially in relation to the pregnancy.
One great place to start looking for Shameless Fiona and OC fanfiction is Archive of Our Own (AO3). There are many stories there that explore different relationships and scenarios involving Fiona and original characters. You can filter by tags like 'Fiona Gallagher' and 'OC' to narrow down the search.
Another idea is that the OC is a long - lost relative of Fiona's. This would open up a whole new set of family dynamics. They could have different views on how to handle the Gallagher family's problems, and there could be a lot of conflict and resolution as they figure out their relationship and how to move forward together.
It could create a contrast. While the Haunting Ground is a place of darkness and danger, Fiona's pregnancy represents new life. This contrast can add depth to the story. The presence of her unborn child might also bring out different sides of the other characters. Some might become more protective, while others might be more malicious in the face of this new hope.
Perhaps an unknown entity from a different realm. Loki might have had an encounter during one of his travels to other worlds, and this being's powers or nature caused the pregnancy. It could be a powerful magical being that Loki underestimated, and now he has to deal with the consequences.
Naruto would experience a huge shift in his identity. He'd go from being the son of Minato (in the original story) to being Minato's father. This would likely make him more responsible and mature. He'd have to think about protecting Minato and guiding him, which is different from his usual role of being the underdog striving for recognition.
A great way would be to include the cultural aspects of the Thundercats. How do they view pregnancy and new life? There could be special ceremonies or traditions that are followed. And it would be interesting to see how Lion - O and Pumyra's relationship is affected by these cultural elements during the pregnancy.
Another option could be an old flame from her past that re - enters her life just as she discovers she's pregnant. This would create a complex love triangle - type situation where she has to decide between her past and her present, all while dealing with the pregnancy.
Another option could be a mysterious figure. For example, a character from another dimension or a new character introduced specifically for this plot twist. This could add an element of mystery and intrigue to the story.