The theme of power dynamics between them is also quite popular. Given that Sadao is a demon and Emi is an angel, their different powers and how they interact with each other in various situations, such as in battles or when they are forced to cooperate, are often explored in fanfiction.
Sometimes, Tumblr can also be a good source. There are many bloggers on Tumblr who are into anime fanfiction. However, the search function might be a bit tricky, so you may need to use specific tags related to the show, the characters Sadao and Emi, and fanfiction.
A common theme is the relationship between Emi and Maou. Whether it's a slow - burn romance or a more platonic friendship evolving, it's often explored.
A prevalent theme is adaptation. The devil has to adapt to the human world in order to survive, and Emi also has to adapt to co - existing with him. Also, the theme of redemption can be seen sometimes. Maybe the devil starts to do good things in the human world, which is a form of redemption for his past actions in their demon world.
One idea could be that Maou and Emi have to team up to face a new, more powerful demon from Maou's past. This demon challenges Maou's leadership and Emi realizes she has to help him to keep the peace in their current world.
The appeal often lies in the relationship between Maou and Emi. Their dynamic, being from opposite sides originally, is very interesting.
Another way is by exploring their pasts. Maybe they discover that they have more in common than they thought in terms of their upbringing in their respective worlds. This newfound knowledge can make them more empathetic towards each other. For instance, if they find out that they both had a mentor figure who had a big impact on their lives, it could create a bond.
One great fanfiction could be 'The Unlikely Alliance' where Emi and Maou have to team up in a different way than in the original story to face a new threat. It has really interesting character development.
One popular fanfiction might be 'Maou and Emi's New Adventure' where they face a new threat in the human world together. It's full of their typical banter and some sweet moments.
One common theme could be role - reversal. Since it's a genderbend, the traditional male and female roles in the original work might be swapped, leading to different power dynamics and relationships.
One idea could be that Emi and Maou have to team up against a new threat in the human world that is more powerful than any they've faced before. Maybe a powerful demon from the other world has found a way to cross over and is causing chaos, forcing Emi and Maou to put aside their differences and use their combined powers to stop it.