I think it could be a combined form of a character. For example, if Issei were to somehow gain the power of the Nine - Tailed Fox. He already has his Boosted Gear and his experience in battles in the Highschool DxD world. Adding the Nine - Tailed Fox's chakra would make him an extremely formidable opponent. He could use the fox's chakra to enhance his Sacred Gear even further, and his new form might have abilities that are a combination of both universes, like using the fox's tails as a weapon while also having the power of the Boosted Gear.
It's hard to say for sure, but Naruto himself could be a strong contender. His vast array of jutsu and his ability to grow in power constantly could make him extremely powerful. He has faced incredibly tough opponents and come out on top in the Naruto universe, so in a merged universe, he could use his skills to take on the devils and other beings.
It could be Issei if he were to gain Naruto's abilities. He already has his own set of powerful skills in the DxD world. Adding Naruto's chakra and jutsus like the Rasengan or the ability to summon toads would make him extremely formidable.
The storylines can be combined by creating a common threat. Suppose an enemy emerges that is interested in both the power of the Bijuu and the power sources in Highschool DxD. This would force the characters from both universes to team up. Also, the cultural differences between the Naruto world and the Highschool DxD world can be explored. Naruto could introduce concepts like ninja clans and shinobi training to the DxD characters, while they could introduce him to the complex social hierarchies and supernatural politics in their world.
One way to combine them could be through a crossover event. Maybe some sort of interdimensional rift opens up and characters from Highschool DxD end up in the Justice League's universe. For example, Issei could find himself in Gotham City and have to team up with Batman to fight off a common enemy. Their different powers and abilities would make for an interesting dynamic. Issei's boosted gear and his ability to use dragon powers could complement Batman's detective skills and martial arts expertise.
Probably Great Red from Highschool DxD. He's already an extremely powerful being in his own universe. If he were to encounter the power of the Omnitrix and all the alien forms in Ben 10, he could potentially assimilate or use those powers in some way to become even more powerful.
It's hard to say for sure, but some might argue that it could be Julius Novachrono from Black Clover. His time - magic is extremely powerful, and he could potentially manipulate time to his advantage in battles against the Highschool DxD characters. He could stop time and land fatal blows before his opponents even realize what's happening.
Well, one way is through character exchanges. Imagine a character from Naruto like Naruto himself ending up in the Highschool DxD universe and meeting Leviathan. He could introduce his ninja skills to that world and also learn about the devil powers. Another way is to have a common enemy. For instance, a new threat that can only be defeated by combining the unique powers from all three, like the Rasengan combined with a Highschool DxD power against an enemy threatening Leviathan.
Naruto fanfictions often add new jutsus or powers to the characters, thus expanding the power system of the universe. For instance, a story might introduce a new type of Sharingan ability. In Highschool DxD, fanfictions can expand on the idea of the Sacred Gears. A fanfic like 'The Sacred Gear Evolution' shows how these gears can have even more complex and powerful forms, which enriches the Highschool DxD universe.
Well, in some fanfictions, Naruto's Nine - Tails power is integrated into the Highschool DxD plot. This can lead to some really intense storylines. Imagine the Nine - Tails' chakra being mistaken for some sort of extremely powerful demonic or angelic energy. It would cause a stir among all the factions in the Highschool DxD world.
A powerful Naruto Highschool DxD fanfiction often has a well - thought - out plot that combines the two universes seamlessly. For instance, it should explain how Naruto's abilities work in the Highschool DxD world and vice versa.