One idea could be to have the characters from High School DxD cross over into a world ruled by different gods. They could be tasked with a quest to restore balance. For example, Issei could use his Boosted Gear in a realm where the gods' powers are waning and his power might be the key to saving the day.
Maybe the gods from different mythologies in the fanfiction could hold a tournament and the High School DxD characters are forced to participate. This would lead to all kinds of battles and character development as they face off against powerful divine opponents. For instance, Rias could use her strategic mind to outwit some of the more brute - force - oriented gods in the tournament.
One idea could be exploring a new Godly power that the main character discovers within the Highschool DxD universe. Maybe it's a power that has been long forgotten and its emergence shakes up the existing power dynamics among the devils, angels, and fallen angels.
Perhaps Natsu's presence could disrupt the balance between the three factions in the Highschool DxD world. His strong sense of justice might lead him to oppose some of the more underhanded dealings among the devils, angels, and fallen angels. This could start a chain of events where he has to prove himself and his beliefs to the various characters.
One idea could be that Inuyasha gets transported to the world of Highschool DxD. He might mistake the devils and angels there for demons from his own world and start attacking them. This would create chaos and also lead to some interesting alliances as the characters from Highschool DxD try to figure out who he is and why he's there. Another plot could involve a shared enemy. Maybe a powerful demon from Inuyasha's world somehow makes its way to the Highschool DxD universe and starts causing trouble for both groups. They would then have to combine their powers and knowledge to defeat it.
One interesting plot could be the characters from Highschool DxD somehow ending up in the Bleach universe. Maybe Issei and his friends are mistaken for Hollows due to their demonic energy and are hunted by Soul Reapers. They then have to prove their innocence while also learning about the complex spiritual world of Bleach.
Well, there could be a plot where the Sacred Gears in Highschool DxD are somehow connected to the ninja powers in Itachi's world. Itachi discovers that some of the students with Sacred Gears have a latent ninja ability within them. He then decides to train these students to help them control and enhance their powers. In return, the students of Highschool DxD help Itachi understand the complex social and power dynamics of their world, which is very different from his own.
A cool plot idea could be a tournament where characters from both Highschool DxD and Naruto participate. The ninjas would show off their jutsu and the devils would display their powers. Also, imagine if Naruto got a Sacred Gear. He could use it to enhance his Rasengan or even create a new jutsu that combines his ninja abilities with the power of the Sacred Gear. Or perhaps, the female characters from Highschool DxD like Rias could teach some of the kunoichi in Naruto about different ways of using their femininity as a power, which could lead to some interesting character development for the kunoichi.
Another plot could involve the Quincy from Bleach. Uryu could be mistaken for a Devil by some of the Highschool DxD characters because of his unique powers. This misunderstanding could lead to a conflict at first, but then they realize they are all on the same side against a greater threat, like a powerful fallen angel who is trying to start a war between the Hollows, Devils and Quincies.
Maybe the dragons from Fairy Tail are related to the dragons in Highschool DxD in some ancient way. So, characters like Igneel might have a connection to the dragon in DxD. This could lead to a search for the truth about their origins and a fight against a common enemy that threatens both worlds.
Maybe in the fanfiction, the Campione characters are invited to the Highschool DxD world to participate in a special tournament. They bring their own unique skills and knowledge, and during the tournament, they form unexpected alliances with the devils, angels, and fallen angels. This would allow for a lot of character interactions and development.
One idea could be that Naruto characters attend the Highschool DxD academy. For example, Naruto himself could be a new student with his unique ninja skills. He might form an unlikely friendship with Issei. Another idea is a battle tournament where ninja techniques are pitted against the devilish powers in the DxD world.