A third idea is a crossover where the characters from both universes have to team up to face a common enemy. Maybe an ancient evil that threatens both the world of Fate and the DxD world. They would need to combine their powers and strategies, like the devils using their demonic powers along with the Servants' unique abilities.
First, you need to have a solid understanding of both universes. Know the rules, the characters, and the powers in Fate and DxD. This will help you create a believable crossover. For instance, if you're bringing a Fate character into the DxD world, you have to consider how their magic will work in a world full of devils and angels.
One idea could be to have the Servants from Fate interact with the Devils in DxD in a battle royale setting. Maybe the Holy Grail somehow ends up in the DxD universe and all the factions are after it.
Shirou from Fate and Kiba from DxD could team up. Shirou's selfless desire to protect others combined with Kiba's sense of justice as a former Exorcist turned Devil would make them a formidable duo. They could share their combat techniques and learn from each other while fighting against the various threats that lurk in a world where the two universes have collided.
Well, DxD crossover fanfiction often combines elements from different universes to create unique storylines. The characters might interact in unexpected ways, and there can be lots of exciting battles and plot twists.
One idea could be to explore how Hercules' incredible strength affects the Fate universe. Maybe he is summoned as a Servant in a Grail War and his brute force challenges the traditional battle strategies of other Servants. His Twelve Labors could also be incorporated in some way, like each Labor being a trial he has to face within the context of the Fate story.
Issei is often a popular character in Fate DxD Gamer fanfiction. His original character traits from DxD can be enhanced or modified in interesting ways due to the gamer elements. For instance, he might gain new leveling systems related to his perverted nature, which could be both humorous and add a new layer to his character development.
One possible fanfiction could be about Naruto characters attending the school in High School DxD. For example, Naruto's positive and determined attitude might clash with the more complex social and power dynamics in the DxD world. He could make new friends or even rivals among the devils, angels, and fallen angels.
One possible fanfiction could be about Naruto characters somehow ending up in the world of High School DxD. Maybe Naruto's skills in combat and his determination would make him a formidable opponent in the battles that often occur in the High School DxD universe. He could form interesting alliances or rivalries with the characters there.
One idea could be that the characters from Highschool DxD end up in the Hellsing universe. Maybe Issei and his friends are mistaken for new types of vampires due to their devil powers. They then have to team up with the Hellsing organization to clear their names and also fight against a common enemy that is threatening both the devil and human worlds in this combined universe.