One popular theme is the 'forbidden love' aspect. Since Mal is from the Isle of the Lost and Ben is the king of Auradon, their relationship can be seen as something that goes against the norm in their world, which makes for an interesting and often explored theme.
One popular theme is their relationship development. Fans love to read about how their love grows and overcomes obstacles. Another is adventure, like them going on quests together. For example, in many fanfictions they might go on a magical adventure to save Auradon or the Isle of the Lost.
One common theme is love conquering all. Their relationship is a central part, and often they face obstacles like different upbringings or outside forces trying to break them apart, but their love prevails. For instance, in a lot of fanfictions, Mal's insecurities about being a former villain are soothed by Ben's love.
Love is a very common theme. Since Mal and Ben are a couple in the Descendants universe, many fanfictions focus on their relationship, whether it's about them falling in love, facing challenges in their relationship, or growing closer. For example, some stories might have them dealing with jealousy from other characters.
One great Mal and Ben fanfiction could be about their adventures after the events of Descendants 2. Maybe they go on a secret mission to another isle to help some lost souls. It could be filled with magic and their growing relationship.
Love is a very common theme. Their relationship is central in the Descendants series, and fanfictions often expand on it. It could be about their first love, how they fell for each other despite their different backgrounds.
Romance is also a big theme. Some fanfictions imagine a romantic relationship between Mal and Audrey. This could involve a slow - burn kind of story where they gradually fall in love, or a more sudden realization of feelings in the midst of some big event in the Descendants world.
An appropriate storyline might be Mal and Ben hosting a big event to celebrate the unity of all the kingdoms in the 'Descendants' world. They could plan it together, deal with various problems that pop up during the planning process, and see how their relationship deepens as they work towards a common goal. It could also involve them inviting characters from all walks of life to participate, which would show their inclusive leadership style.
A cool fanfiction could be Mal and Ben swapping powers for a day. This would lead to all sorts of comical and also challenging situations. For instance, Mal would have to deal with the responsibilities of being a king - like making important decisions in court - while having Ben's more 'goody - two - shoes' power. And Ben would be on the Isle of the Lost, trying to use Mal's magic in a place where it's not always welcome. Or a story where they go on a secret adventure away from Auradon, just the two of them, and discover a hidden land that has its own set of rules and magic, and they have to adapt quickly to survive.
You could write a fanfiction where Ben and Mal get transported to an alternate dimension. In this dimension, their roles are reversed. Mal is the one in charge of Auradon and Ben is from the Isle of the Lost. They have to figure out how to get back to their own world while also learning about each other in this new dynamic.
One idea for a fanfiction could be about how Mal and Ben's baby selves might interact in a magical nursery. Maybe there are enchanted toys that reflect their future selves. For example, Mal's toy dragon could be a sign of her future connection to Maleficent's powers.