Yes, there are quite a few. You can find them on fanfiction platforms like Archive of Our Own. Some might focus on their adventures together in the Descendants universe, exploring their relationship in different scenarios, like teaming up against a new threat or just getting to know each other better on a personal level.
The 'forbidden love' trope is quite common. Since they come from different worlds in the Descendants universe, their relationship might be seen as unacceptable by some. Also, the 'character growth' trope. Through their interactions, both Harry and Mal grow as characters. For example, Mal might learn to be more spontaneous from Harry, and Harry could learn more about loyalty and friendship from Mal. There are also fanfics that use the 'fish out of water' trope when they visit each other's worlds.
One common theme is the exploration of their friendship. Since they come from different backgrounds, it's interesting to see how they bridge those gaps. For example, Mal being from one group and Harry from another.
One popular Mal and Harry fanfic might be 'Mal's New Adventure with Harry'. It could explore their relationship in new and exciting ways, like them going on a magical quest together. Another could be 'Mal and Harry: A Different Bond' which might focus on how their different personalities complement each other in the story. There are also fanfics that might be set in alternate universes, such as 'Mal and Harry in a Modern World' where they have to deal with modern - day problems while still having their magical connection.
One common theme is the clash of different worlds. Harry comes from the wizarding world and Mal from the Descendants world, so there's often exploration of how their different cultures and powers interact.
One reason could be that Harry is more exciting. Ben is a bit too proper and goody - two - shoes for Mal sometimes. Harry has that edge of danger that Mal might find more appealing.
You can also try Wattpad. While it has a broader range of content, there are often some hidden gems of Mal and Harry fanfic there. Some Tumblr blogs dedicated to the fandom might also reblog or write their own fanfics. You can follow these blogs and keep an eye out for any new Mal and Harry stories. Another option is to ask other fans in the community. They might be able to recommend some really good fanfics that they have read and enjoyed.
One possible plot could be that Mal and Harry face challenges in raising their baby in a world full of magic and chaos. They might have to deal with villains who want to use the baby's power for their own gain. Also, their friends from the Isle of the Lost and Auradon could either help or create more complications.
A fanfiction might explore how Harry and Mal's relationship evolves after the events of Descendants 2. For example, Mal, now more in touch with her good side, tries to convince Harry to also look for a better way of life. Harry, initially resistant, slowly starts to see the value in what Mal is saying. Their interactions could be filled with tension, humor, and moments of real connection as they navigate their changing identities.
Well, it could start with Mal realizing that Harry has a wild and free spirit that she's drawn to. Maybe Harry shows her a side of the Isle that Ben never could. He might take her on adventures that are more exciting and less rule - bound compared to the life she'd have with Ben.