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.
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.
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.
The main characters are clearly Jonas and Fiona. They are likely the focus of the story as the title implies that they are the ones who are pregnant (in the context of the fanfiction).
It could also be about the reactions of their friends and family. Some might be supportive, while others could be against it due to the norms of their world. The story could then focus on how Jonas and Fiona deal with the different reactions and still stay true to their decision to have the baby.
It would likely become more intense and protective. Jonas would be extremely protective of Fiona and the unborn child, given his experiences and the knowledge he has. He would be determined to keep them safe from any harm that the community might pose.
Another plot idea is that Fiona's pregnancy is not a normal one. There could be some sort of dark magic involved. As she progresses through her pregnancy, she has to not only deal with the regular threats of the Haunting Ground but also figure out what is wrong with her pregnancy. She might meet a mysterious figure who claims to know how to save her and the baby, but she has to trust them and complete dangerous tasks.
One common theme could be the challenges Lip faces in dealing with the pregnancy situation. Given his character's background and the nature of the Shameless universe, it might involve struggles with family reactions.
One possible plot could be that they face opposition from the community. Since the community in 'The Giver' is so strictly controlled, their pregnancy might be seen as a deviation from the norm. They could be forced to go into hiding to protect their unborn child from the authorities who might want to take it away or 'correct' the situation.