One idea could be a crossover where a character from 'Harry Potter' accidentally stumbles into the world of 'Devils and Realist'. For example, Harry Potter himself could find himself in the midst of the political intrigue in the 'Devils and Realist' world, having to use his magic in new and unexpected ways to survive and figure out how to get back home. Maybe he forms an unlikely alliance with one of the main characters from 'Devils and Realist' who is intrigued by his magic powers.
The magic in 'Harry Potter' is mainly based on spells, potions, and magical artifacts. In 'Devils and Realist', there are different types of powers related to demons and the political power plays. In a fanfic, one could combine them by having a spell that can summon or control a demon from 'Devils and Realist'. For example, a powerful wizard in the 'Harry Potter' world discovers an ancient spell that links to the demon realm in 'Devils and Realist' and starts to experiment with it, but quickly realizes the danger as the demons have their own motives and power hierarchies.
The ending of Devils Line is intense. It brings closure to some storylines while leaving others open-ended, creating a bittersweet conclusion for fans.