A possible plot in highschool dxd gremory bashing fanfiction is the exploration of the Gremory family's past secrets. Maybe there are dark secrets that, if exposed, could damage their reputation and power. A fanfiction could revolve around a character who discovers these secrets and decides to use them against the Gremory family. Or perhaps a situation where the Gremory family is faced with a new type of enemy that specifically targets their weaknesses, which are revealed as the story progresses.
One interesting crossover could be the sharing of combat techniques. For example, Naruto's shadow clone jutsu might be used in the Highschool DxD universe in a creative way during battles. It could help in confusing opponents just like how it does in the Naruto world.
Issei is often involved. Since he has a connection to the Gremory family through Rias, his actions and decisions can sometimes be a part of the Gremory - bashing scenarios. Another could be Sona, as her family has a relationship with the Gremorys and she might be in a position where she has to take a stance against them in some fan - created situations.
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.
The Highschool DxD manga has indeed finished. It wrapped up after completing all the planned story arcs and providing a satisfying conclusion for the readers.
One possible idea for this fanfiction could be Naruto being transported to the world of High School DxD. He might end up attending the same school as Rias. Given Naruto's strong sense of justice and his combat skills, he could quickly become involved in the supernatural battles and power struggles in that world. He might also form an interesting relationship with Rias, perhaps starting as friends and gradually developing into something more as they face various challenges together.