One possible plot could be that Ichigo somehow gets transported to the world of High School DxD. He might initially be seen as a threat by the devils and angels there due to his unknown powers. But as they learn about his motives of protecting people, they start to work with him. Ichigo could also teach the characters in High School DxD about his unique fighting style and the concept of Hollow and Soul Reaper powers, leading to new and exciting battles against common enemies.
Perhaps in this crossover, there is a powerful enemy that neither the Ichigo's world nor the High School DxD world can defeat alone. So, Ichigo teams up with the various factions in High School DxD. They combine their powers and knowledge. For example, Ichigo's Bankai could be combined with the Sacred Gear powers of the characters in High School DxD to create a super - powerful attack against the common foe. This would lead to interesting character interactions as they learn from each other's strengths and weaknesses.
A plot could involve Ichigo being mistaken for a new type of devil in the High School DxD universe. The devils might try to recruit him while the angels see him as a potential danger. Meanwhile, Ichigo is just trying to find a way back home. Along the way, he befriends some of the characters like Issei and helps them in their battles against the fallen angels, all while trying to keep his own identity and powers a secret.
One possible interesting plot could be Sasuke arriving in the Highschool DxD world during a time of chaos. Maybe there's a new threat that neither the devils nor the other factions can handle easily. Sasuke's ninja skills and Sharingan would give him an edge in combat. He could form an alliance with Issei and his group, teaching them some ninja techniques in return for learning about the supernatural powers in their world.
Well, a cool plot might involve Issei from Highschool DxD being sent to the Naruto world. He could learn ninjutsu there and then bring some of those skills back to his own world. He might befriend Naruto and they could work together to fight a common enemy that has somehow crossed between the two worlds, like an evil group that wants to take over both the ninja world and the world of devils and angels.
In some fanfics, a great plot twist is when Ichigo's Hollow powers are misinterpreted by the DxD characters as some sort of devilish power. This leads to a lot of misunderstandings and conflict at first. Another interesting plot twist can be when Ichigo somehow ends up being related to an important figure in the DxD world through some convoluted backstory.
One possibility could be that Ichigo's powers would make him a very unique and powerful character in the Highschool DxD universe. His zanpakuto abilities and high - level combat skills could give him an edge in the battles against the devils and other supernatural beings in that world.
The plot might involve Ichigo accidentally stumbling into the Highschool DxD universe while on a mission in the Soul Society. His godlike presence could disrupt the balance between the devils, angels, and fallen angels. He might be seen as a threat at first, but as they learn more about him and his noble intentions, they could start to work with him to face a common enemy that is emerging due to the new presence in their world.
The plot could involve Ichigo's unique abilities causing a stir in the Highschool DxD world. Since he has the power of Mugetsu, he could be a target for some groups who want to either use his power or get rid of him. And at the same time, he could befriend some of the characters from DxD and they could learn from each other's powers.
One possible plot could be that Harry Potter, while on a new magical adventure, somehow gets transported to the world of Highschool DxD. He could use his unique magic skills to help the characters in Highschool DxD deal with their own supernatural threats. For example, his Patronus charm might be effective against some of the evil creatures in the DxD world.
One interesting plot could be Naruto being summoned to the world of Highschool DxD. His unique ninja skills and chakra would be a huge shock to the devils, angels, and fallen angels in that world. He could end up forming an unlikely alliance with Issei, perhaps teaching him some of his ninja techniques in exchange for learning about the Sacred Gears. They could then face a common enemy together, like a powerful devil who wants to use Naruto's power for his own evil plans.
A cool plot could be that the Highschool DxD characters are transported to the One Piece world. Issei, being the perverted yet brave character he is, might be really excited to see all the different female characters in One Piece. But he also realizes that they are in a dangerous world. Meanwhile, the One Piece characters are baffled by the strange powers and personalities of the Highschool DxD crew. They all have to work together to face the various pirates and threats in the One Piece world.
The plot could involve Ben's aliens having an unexpected effect on the magic in the Highschool DxD world. His alien DNA might interact with the devil or angel powers in strange ways. For instance, if he transformed into Heatblast near a magic spell, it could cause the spell to go haywire. And his presence could also disrupt the power balance between the different factions in the DxD world, leading to new alliances and rivalries.