Another way is to create a character who is an outsider in both universes. Maybe a human from our world who gets sucked into the crossover world. This character could bring a fresh perspective as they try to understand the rules of both Devilman Crybaby and the other franchise. They could also have a unique power that comes from their 'outsider' status, like being able to nullify demonic or other special powers temporarily.
You can combine the characteristics of characters from both franchises. For example, if you're crossing with One Piece, take the Devil Fruit powers and combine them with the demonic powers in Devilman Crybaby. A character could have the power of a Zoan - type Devil Fruit and also be a Devilman, having the ability to transform into a powerful hybrid form.
Look at the themes of both franchises and create a character that embodies those themes in a new way. In a crossover with Fullmetal Alchemist, you could create a character who has lost something in the Devilman Crybaby battles (like a loved one to a demon) and is now trying to use alchemy to bring them back. But this goes against the laws of alchemy in the FMA world, creating an interesting conflict within the character.
Another plot idea is to crossover with Neon Genesis Evangelion. The Angels in Evangelion could be some sort of new form of demons in the Devilman Crybaby universe. The Eva pilots might have to team up with Devilmen to fight these powerful threats. It could explore the psychological aspects of both universes as the characters deal with the new and terrifying situation.
There is also 'Devilman Crybaby: A New Dawn'. This fanfic focuses on what could have happened if the events at the end of the series had a different outcome. It's a really creative take as it imagines a world where some of the main characters make different choices, leading to a whole new set of adventures and challenges.
Appearance matters too. You could give your OC a unique look. Maybe they have heterochromia (two different - colored eyes) which is a sign of their connection to both the human and devil worlds. Their clothing style could also be a mix of modern and ancient, representing the blend of cultures in the Devilman Crybaby universe.
There's a fanfic titled 'Akira and Ryo: A Different Path'. In this story, it takes a more alternate universe approach. Instead of the tragic events in the original, they end up on a journey together to discover a new way to coexist with the demons. It's a really interesting take on their relationship as it shows them growing closer in a more positive environment.
In a crossover with My Hero Academia, characters like All Might could be included. His sense of justice would make him an interesting counterpart to Devilman. He could be initially skeptical of Devilman's powers but then come to respect him as they fight together against a villain that combines the traits of demons and quirks.
One reason is the rich world of Devilman Crybaby. The original series has complex characters and a dark, engaging story. Fanfic writers can expand on this. For example, they can explore the relationships between characters like Akira and Ryo in more detail.
Sure. You can try looking on fanfiction websites like Archive of Our Own. There are often many works exploring the relationship between Akira and Ryo in Devilman Crybaby. Just search for 'akira x ryo devilman crybaby' on the site and you'll likely find a good selection.
One idea could be a crossover with Neon Genesis Evangelion. Devilman and the Eva pilots could team up to fight a common enemy that threatens both their worlds. Maybe a new type of demon that has the ability to control Angels.
Devilman Crybaby has a corresponding manga. The anime takes inspiration from it, but there might be some differences in the way the story is presented in each medium.
In the 'devilman crybaby origin story', a young man gets involved in a situation where he merges with a demon, which results in him becoming Devilman. This event is the starting point of his new life filled with challenges and battles.