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.
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.
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 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.
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.
A third idea is that there's a mix - up with the curses in both worlds. Ranma's curse of changing into a girl when splashed with cold water gets combined with some of the magic in Highschool DxD. This creates a whole new set of problems and adventures for him as he tries to figure out how to deal with this strange new situation while also trying to keep his identity a secret from the new characters he meets.
Another concept is a more comical crossover. Shirou tries to adapt to the different rules and magic in the Fate Apocrypha world. He constantly gets into funny situations as he misunderstands the local customs and magic systems. For example, he tries to cook using Apocrypha - style magic ingredients and chaos ensues.
One idea could be Naruto being summoned into the High School DxD world during a battle. His unique ninja skills would shock the devils and angels in that universe. He might form an unlikely alliance with Issei, perhaps teaching him some ninja techniques in return for learning about the Sacred Gears.
One idea could be that Naruto is somehow transported to the world of High School DxD. He could use his ninja skills, like shadow clones, in the battles that take place in the DxD universe. Maybe he'd form an alliance with Issei and his group, and teach them some ninja techniques to fight against the devils and fallen angels.
Another possibility is a story where the power - up systems of both Naruto and High School DxD are combined. For example, Naruto could learn about the Boosted Gear and use it in a new form of his Rasengan. Or Issei could gain chakra and learn some basic ninja techniques. This would create an interesting dynamic as they face common enemies.
One idea could be that Naruto is somehow transported to the world of High School DxD. He could use his ninja skills, like shadow clones, in the battles against the devils and fallen angels in that universe. Maybe he'd form an alliance with Issei and his group, teaching them some unique ninja techniques.