Friendship or alliance is also a major theme. Since they are from different franchises, the idea of them coming together as allies or friends is interesting. Fanfictions might show how they learn to trust each other, overcome initial differences, and work towards a common goal. It could also involve some character development for both as they interact more with each other.
Well, one place to look for 'Cutie Honey x Devilman' fanfiction is Archive of Our Own (AO3). There are various works there that combine the two universes in interesting ways. Some might focus on a shared adventure where Cutie Honey and Devilman team up against a common enemy, with unique takes on their powers and personalities interacting.
The story of 'Honey x Honey Drop' focuses on the blossoming relationship between the main characters. It's popular because it offers a delightful escape into a world of romance and fantasy. The well-developed characters and beautiful art contribute to its appeal.