One possible plot could be that Clary and Jace face a lot of social stigma. Their friends might be shocked and some might even turn their backs on them. They would have to deal with their families' reactions, which could range from disappointment to support. They'd also have to figure out how to continue their education while preparing for a baby.
It would change in many ways. They would go from being carefree high school sweethearts to having a lot of responsibilities. There would be more arguments as the stress of the situation builds up. But at the same time, they would also have deeper moments of love and understanding as they face this huge life change together. Jace might become more overprotective of Clary, and Clary would need Jace's support both emotionally and physically.
In this fanfic, the pregnancy could be a sign of hope for the Shadowhunter community. Clary and Jace, being two important figures, are now having a child. Their journey could involve seeking ancient knowledge from the Silent Brothers or the Iron Sisters about how to ensure the health of the baby, considering their unique genetic makeup as Shadowhunters. Along the way, they could strengthen their relationship even more as they face the challenges together.
Their first interaction could be intense. Maybe Jace, with his confident and somewhat brooding charm, catches Clary's eye in the hallway. She might be intrigued by his cool exterior. He could give her a lingering look, and she might blush. Then, he might say something cryptic like 'You seem different from the others here.'
Jace will probably face a lot of guilt. His cheating not only hurt Clary but also has implications for the unborn child. He might find himself ostracized by their friends who side with Clary. Also, his reputation within their community in the fanfiction world could be severely damaged. If he realizes his mistake and wants to make amends, he'll have a long and difficult road ahead to earn Clary's forgiveness and trust again.
One typical challenge is secrecy. In some fanfictions, their families or certain school groups might not approve of their relationship. So they have to keep it a secret, which is hard when they're in high school and constantly around others.
In this fanfiction, the development might involve Clary's friends trying to reunite her with Jace. They could see that Jace had his reasons for leaving and try to make Clary understand. On the other hand, Clary could be so hurt that she refuses to listen. Meanwhile, the pregnancy progresses, and Clary has to make decisions about the baby's future, like whether she wants to raise it alone or consider adoption. There could be a lot of emotional turmoil and growth for all the characters involved.
It might progress slowly at first. They could start as friends, sharing lunches and talking about their classes. Jace might gradually open up to Clary about his dreams and fears. For instance, he could tell her how he feels pressured to be the best in everything. As time passes, they become closer, and their friendship starts to turn into something more, like when they hold hands for the first time during a school event.
One possible plot could be that Jace and Clary are trying to protect their baby from a new, unknown demon threat. They have to go into hiding and rely on their friends from the Institute to help them. Along the way, they discover new powers within themselves as they are more determined than ever to keep their child safe.
In such a fanfiction, perhaps Clary would have to face the challenges of pregnancy alone at first. She might struggle with her emotions, feeling abandoned by Jace.
The relationship would evolve significantly. Before the baby, they were mainly focused on their own adventures and relationship. But now, they are parents. They would need to support each other in different ways. For instance, Jace might be more protective of Clary and the baby. Clary would be more nurturing, and they would both have to learn to balance their duties as Shadowhunters with their new family life.