Well, one possible plot could be that Natsu is betrayed in his own world and somehow ends up in the DxD Highschool universe. Maybe he has to deal with the different power dynamics and politics there while also trying to figure out who betrayed him in his original world. He might form new alliances with the characters in DxD Highschool and use his unique abilities to survive and seek revenge for his betrayal.
Natsu, being a fiery and passionate character, would likely have a rather intense interaction with the DxD Highschool characters. He might be initially distrustful due to his recent betrayal. But some of the more friendly characters in DxD, like Issei's group, could try to befriend him. Natsu would probably show off his powers a bit, which would either impress or intimidate the others. And there could be some clashes in personalities, for example, Natsu's straightforwardness against some of the more devious DxD characters.
One idea could be a crossover where Natsu finds himself in the world of High School DxD and has to deal with the devils and angels there. He might use his fire magic in unique ways to combat the various threats in that universe.
One thing that makes this type of fanfiction appealing is the contrast. Naruto, a character known for his loyalty, is betrayed in his own world. Then he's thrown into the DxD highschool where there's a whole new set of rules and relationships. The harem part means there are multiple characters with different personalities that can have various interactions with Naruto. It's also about seeing Naruto grow and change in this new environment, perhaps learning to trust again through his relationships with the harem members.
Incorporate some mystery elements. Maybe there's a strange force at work in the combined world of Natsu and High School DxD that is causing chaos. Natsu and the main characters from High School DxD have to work together to figure out what it is. This will make the readers curious and keep them engaged throughout the story.
I'm sorry, but I can't recommend lemon (adult - themed) fanfiction as it goes against ethical and often site - use policies. However, you can try looking for regular Natsu x Highschool DxD fanfiction on popular fanfiction platforms like Fanfiction.net or Archive of Our Own. There you might find some well - written stories about the crossover that are suitable for all audiences.
One interesting 'Betrayed Highschool DxD Fanfiction' could be where the main character is betrayed by his friends in the Occult Research Club and has to find a new path on his own. Maybe he teams up with some unexpected allies from the underworld.
Issei is often a central character in these fanfictions. Since he is the main protagonist in the original series, his relationships and potential betrayals are explored. For example, a situation where his friends or allies in the Occult Research Club turn against him for some reason, like jealousy over his growing power or a misunderstanding about his actions.
One interesting plot could be where a main character is betrayed by a close friend within the Highschool DxD universe. For example, a devil might be betrayed by another devil who has a hidden agenda related to power or territory. This could lead to intense battles and power struggles as the betrayed character tries to seek revenge and regain their status.
One plot idea could be Natsu accidentally ending up in the Highschool DxD world and having to adapt to the different rules and magic systems there. He could make friends with some of the main characters and they could team up to fight a common enemy that threatens both his world and the Highschool DxD world. Another idea is that Natsu's unique brand of magic could be a mystery to the characters in Highschool DxD and they try to figure out how it works while also teaching him about their own powers.