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.
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.
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.
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.
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.
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 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.
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.
Meredith could be an important side character. Given her history with Derek, there could be some tense moments as she witnesses Addison's pregnancy. It could add an element of drama to the story.
Obviously, Addy Carver herself is the main character. Then there could be the father of the baby. He could be an old flame, a new acquaintance, or someone she has a complicated relationship with. There might also be her friends, who could be either very supportive or cause some drama because of her pregnancy. Her family members, like her parents or siblings, would also be potential characters, each with their own reactions to her situation.
Ino could be a great supporting character. As Sakura's friend, she could offer emotional support, perhaps share her own experiences or just be there to listen. Kakashi could also play a role. He might offer some sage advice on balancing family and ninja duties, drawing from his own experiences. And of course, Naruto. He could be the one to bring some cheer and positive energy, and maybe even offer some practical help like getting hard - to - find supplies for the baby.