In this fanfiction, a powerful figure in the society who has strict ideas about family and class could be an antagonist. Since Marius and Cosette come from different backgrounds, this person might not approve of their relationship and now their new baby. They could try to use their influence to separate the family or at least make their lives difficult by imposing unfair rules or restrictions on them.
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.
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.
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.
Dear Cosette I hope you can accept this love letter because I love you deeply. When I first saw you, I was attracted by your beauty and kindness. Your smile, your voice, everything about you makes me unable to extricate myself. I know there are many obstacles between us, but I believe that as long as we truly love each other, these difficulties will be overcome. I'm willing to give you everything, including my time, my money, and my hard work. I'm willing to go through every obstacle with you and create a beautiful life that belongs to us. Cosette, I love you. This is not just a slogan, but the most genuine feeling in my heart. I hope we can be together and love each other forever. Love, Marius
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.
It could be some of the old enemies from the Sailor Moon series who see the baby as a threat. Maybe the Death Busters, if they were still around, would want to use the baby's power for their own dark purposes.
A group of mischievous animals could also be antagonists. They might be jealous of the attention the baby gets from all the other animals in the neighborhood. So, they start playing pranks on Alvin and Brittany, like stealing the baby's toys or making a mess in their yard. But in the end, they could learn to accept the baby and become friends.