A really engaging storyline could be about Harry Phenex bringing in some unique magic or abilities from his own background into the Highschool DxD universe. Maybe he has a special type of fire magic due to his Phenex heritage that no one in the DxD world has seen before. This could make him a target for some who want to use his power, and also a valuable ally for others. He would have to navigate through the complex politics of the school while also protecting his secret.
One interesting storyline could be Harry Phenex's adventures in the Highschool DxD world while trying to hide his true identity as a Phenex. He might have to deal with the various factions in the school and their power struggles.
There might be a fanfiction where Riser Phenex discovers a new power within himself. This could lead to a whole new set of adventures as he tries to master it and also deal with the political implications within the Highschool DxD universe, like how other devil families react to his newfound strength.
One interesting crossover could be the sharing of power concepts. In Highschool DxD, the Phenex family has unique powers like immortality and fire abilities. In Naruto, there are also various powerful jutsus. A crossover could explore a character from Naruto learning about the Phenex powers and incorporating them into his own fighting style. For example, Sasuke might be intrigued by the Phenex's fire powers and try to combine it with his own fire - based jutsus like the Great Fireball Technique to create a more powerful attack.
Often, the characters' personalities play a big role. Naruto's determination could inspire some of the more laid - back characters in Highschool DxD. And the devils' more sly and cunning nature might teach some of the Naruto characters new ways of approaching problems. There could be a fanfiction where Naruto teams up with Issei to take on a common enemy. They would combine their combat styles, with Naruto using his shadow clones and Issei his boosted gear, creating a really cool fight sequence.
A fanfiction might explore a story where the Occult Research Club members are all transported to a different dimension. In this new world, they have to find a way back home while facing new enemies and challenges. Rias could use her leadership skills in a different way, and Kiba might discover new powers within himself to protect the group. This would also test the bonds between the characters in a whole new setting.
Harry Phenex himself is often a very popular character. Since he is the main focus in this fanfiction concept, his unique background as a Phenex in the Highschool DxD setting makes him stand out.
Another could be Mittelt forming an unlikely friendship with a character from a different faction in the Highschool DxD universe. For example, she befriends a member of the fallen angels' group that her side is usually at odds with. This could lead to all sorts of plot twists as they try to bridge the gap between their two groups. And it would be really cool to see how Mittelt grows and changes through this relationship.
Well, a great storyline could involve a slave character who has a unique relationship with one of the main characters from Highschool Dxd. Maybe they start off as just a slave but through shared experiences, like fighting together against a common enemy, they develop a deep bond that changes the whole dynamic of the story and challenges the traditional master - slave relationship in that world.
One interesting storyline could be about a new character who joins the school and has a unique power that changes the power dynamics among the existing groups. This new character might start as an outcast but gradually earns the respect of the others through their actions.
Another possible storyline is a love triangle between a Saitama Highschool student, a character from DxD, and another student from their own school. This would add an element of drama and emotion to the fanfiction, as they navigate their feelings while also dealing with the supernatural elements of the DxD world.
One interesting storyline could be about a new character who has a unique power that shakes up the power dynamics in the Highschool DxD world. For example, a character with the ability to nullify all magic for a short time, creating new challenges and alliances.