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.
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.
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 common theme is loyalty. Ashiya is always loyal to Maou in the series and fanfictions often explore this aspect further, showing how their loyalty to each other helps them face various challenges.
The appeal often lies in the unique dynamic between Suzuno and Maou. They come from different backgrounds, with Suzuno being a more traditional character and Maou having his devilish charm. Their interactions can be both comical and full of underlying tension.
One great Suzuno x Maou fanfic could be 'A New Beginning' where it explores their relationship in a modern - day setting with Suzuno slowly realizing her true feelings for Maou. It has some really sweet moments between them.
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.
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.