A good fanfic in this regard would have well - developed characters. The author should stay true to the essence of Clary and Jace as they are in the original work. Their personalities, quirks, and insecurities should be on full display.
Clary and Jace's love is a complex and passionate one. They meet in a world filled with shadowhunters and demons. Their initial attraction is immediate, drawn to each other's strength and beauty. However, they face many obstacles. There are secrets about their families and their own identities that keep getting in the way. But through all the trials, their love endures as they fight side by side against the forces of evil.
You can try Archive of Our Own (AO3). It has a large collection of fanfictions related to Jace and Clary's love. There are various tags you can use to narrow down your search, like 'Jace x Clary' or 'Jace Herondale/Clary Fray' to find exactly what you're looking for.
One popular one could be 'Jace and Clary's Unending Love' where they face various challenges together in the Shadowhunter world and their love deepens through it. Another might be 'Love Beyond the Runes' which focuses on their love defying the rules of the Clave.
Their relationship would become more centered around the baby. They would have to make decisions together for the sake of their child, which would strengthen their bond.
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.
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.
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 and Clary love fanfics are popular because their relationship in the original story is so complex and full of passion. Fans want to explore more of their love, add their own twists and turns. It gives fans a chance to delve deeper into the characters' emotions.