Maybe some of the old foes from their past could resurface as antagonists. For example, those who were against Marius and Cosette being together in the first place might see the new baby as a way to cause trouble. They could try to spread rumors or create difficulties for the new family.
One possible plot could be that Marius and Cosette are new parents and they are adjusting to life with a baby. They face challenges like sleepless nights and learning how to take care of the baby's needs. But through it all, their love for each other and for their new child grows stronger.
Perhaps in the fanfiction, Cosette discovers she's pregnant at a time when there are political unrests in their city. Marius is involved in some sort of movement, and he has to balance his duties towards the cause and his family. Cosette, on the other hand, is worried about the safety of their unborn child. They have to rely on their friends and each other's love to get through this tough time, and the pregnancy becomes a symbol of hope in the midst of chaos.
Another theme could be the exploration of their individual growth within the relationship. For example, how Marius becomes more understanding of Cosette's past, and how Cosette becomes more confident in their relationship. It's about how they change and adapt to each other.
One potential side character could be the Thenardiers. They might try to take advantage of Cosette's pregnancy, thinking they can get some money or favors from Marius and Cosette. Madame Thenardier could be especially devious in her plans. Another side character could be a kind midwife in the neighborhood. She would help Cosette throughout her pregnancy, giving her advice on health and child - rearing. And of course, Gavroche, Marius's little friend, could be around, bringing some cheer and a sense of innocence to the story.
A good plot could revolve around Cosette's pregnancy cravings and how Marius tries his best to fulfill them. It could also show how they deal with the financial aspect of having a baby. Maybe Marius takes on extra work, and they learn to budget together.
Fanfiction.net is also a good place. Just search for 'Cosette and Marius' in the search bar. Some fanfictions might be really well - written and give you new insights into these characters. However, the quality can vary a lot, so you might need to read through a few to find the really good ones.
Show their increased closeness. Marius might be more protective of Cosette now. He could accompany her everywhere, from doctor visits to simple walks in the park.
One common theme is immortality. Since Marius is a vampire, his eternal life and how he copes with it is often explored. For example, he might face bouts of loneliness or have to constantly adapt to new time periods.