It could be one of the well - known kamen riders like Kamen Rider Decade. Since he has the ability to travel through different worlds, it would be plausible for him to end up in the world of Highschool DxD. He could use his powers to adapt to the new environment and deal with the various challenges there.
Maybe a new kamen rider character could be created specifically for this fanfiction. This rider could be a half - devil or half - angel, which would make for an interesting connection to the Highschool DxD universe. He would have to balance his two sides while dealing with the complex politics and battles in the school.
Well, it could be about a kamen rider who enrolls in the highschool from DxD. Maybe he has to use his kamen rider powers to fight off devils or other supernatural threats that target the school. He could also form an unlikely alliance with some of the characters from DxD, like Issei and his friends.
There are quite a few out there. Some focus on how the power - ups in Kamen Rider could be translated into the DxD power system. For example, a fanfic might have the Kamen Rider's transformation devices being mistaken for powerful artifacts in the DxD world, leading to all sorts of comical and action - filled situations. You can usually find these on popular fanfiction websites by using relevant tags.
In some fanfictions, the merge happens when a powerful artifact from either universe is activated. This could be a Kamen Rider's belt or a powerful relic from Highschool DxD. When this happens, it creates a rift that pulls elements from one universe into the other. The characters then have to deal with the new and unexpected situations. For instance, the devils in DxD might be intrigued by the Kamen Rider technology and try to harness it for their own purposes.
Issei could be an interesting main character in a 'Highschool DxD Akatsuki' fanfiction. He already has his own journey in the Highschool DxD world, and if he were to interact with the Akatsuki, it would add a whole new dimension to his story. He could learn new techniques from them or even form an alliance against a common enemy.
A new character could be introduced as the main character. For instance, a transfer student who has this hypnotic power. He could be mysterious at first, and as he gets involved with the harem, he gradually reveals his true self and the reasons behind his power.
Issei would be a popular choice. Since he is the main character in the original series, having him time travel would create many interesting storylines. He could use his knowledge of future events and his growth as a character to change things in the past.
It could be an original character created just for the fanfiction. Maybe a nerdy gamer who has always dreamed of being in a world like Highschool DxD and suddenly finds himself there with incredible powers.
In my opinion, Ultraman could be a great character to crossover with Kamen Rider Agito. They both have their own unique powers and fighting styles. Ultraman's large - scale battles against giant monsters could contrast with Agito's more street - level combat against the Unknown. And they could combine their powers in some creative ways, like Agito using his form - changing abilities to adapt to Ultraman's size when they need to fight a really huge enemy together.
One great Highschool DxD and Ghost Rider fanfiction is 'The Devil's Rider'. It combines the supernatural elements of both universes really well. The main character, who has traits of both a Devil from Highschool DxD and Ghost Rider, goes on exciting adventures fighting various demons and devils. It's full of action -packed scenes and some interesting character interactions.
Maybe a rogue ninja from Gaara's world who somehow follows him into the DxD highschool. This ninja could have a vendetta against Gaara and use the chaos of the DxD world to try and take him down. They could also team up with some of the devils who dislike Gaara's presence to create more trouble for him.