Clary might react with a sense of numbness at first. She's pregnant, which is a huge life change on its own, and then to find out Jace has cheated. She could withdraw into herself, not really knowing how to handle it. As time passes, she'll likely start to feel a deep sadness. She might cry a lot and feel like she's lost not only her relationship but also the future she had envisioned with Jace. However, her pregnancy could also give her the strength to be more independent and start thinking about building a new life for herself and the baby.
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.
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.
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.
There's also 'Expecting the Unexpected'. In this fanfiction, it not only focuses on Clary's pregnancy but also how the Shadowhunter world around them reacts. It has some really interesting plot twists and character developments.
You can try Archive of Our Own (AO3). It has a vast collection of fanfictions, and there are likely to be several 'Clary Jace pregnant' ones there. Just use the search bar and type in relevant keywords.
One plot twist could be that one of them loses their memory. For example, Jace might lose his memories of Clary and their relationship, and Clary has to work hard to make him fall in love with her all over again. Another twist could be that there's a hidden prophecy that says their love will bring destruction, so they have to figure out a way to be together without causing chaos. Or maybe a new character shows up who claims to be Jace's long - lost sibling and tries to break them up.
One interesting aspect could be how they overcome the various challenges in the fanfiction. For example, they might have to deal with differences in their magical abilities or the disapproval of other characters. Their love could be a catalyst for change in the world of the fanfiction.
Jace in lemon fanfiction can be quite passionate. His love for Clary is shown in a very intense way. He might be more forward in expressing his desires compared to in the original story. Clary is often depicted as being a bit more vulnerable in these fanfictions when it comes to her relationship with Jace, yet still maintaining her core of courage. She may struggle with her new - found feelings and the physical aspect of their relationship at times, but she grows through it. Overall, their relationship in lemon fanfiction is usually more steamy and emotional.
I don't think it's appropriate. Fanfiction is a great way to expand on the world and characters, but when it comes to sexual content, it crosses a line. The characters of Jace and Clary have a complex relationship in the original story that is more about their fight against evil, their family ties, and their growing love in a more innocent and pure sense. Sexualizing them in fanfiction detracts from what makes them great characters.