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 possible plot could be that Natsu, being in the DxD High School setting, is betrayed by someone he considered a close friend. Maybe this friend reveals Natsu's secrets to the enemies in the DxD world. As a result, Natsu has to face new challenges alone, all while trying to figure out how to deal with the betrayal and regain his trust in others.
Well, Mirajane could be part of the harem. She has a motherly side and would want to comfort Natsu if he was betrayed. Levy is also smart and resourceful, and she could use her skills to help Natsu out. Juvia, with her obsessive love for Gray, might transfer some of that intense loyalty to Natsu if she saw him in a desperate situation after being betrayed.
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 idea could be that Natsu is betrayed by his closest friends in the guild, and in his time of despair, a group of female characters form a harem around him to help him heal and regain his confidence. For example, Lucy could be the first to reach out, using her celestial magic to show him there are still things to fight for. Another plot could involve Natsu being betrayed by a powerful dark mage who manipulates his guild against him. The harem, which might include Erza, Wendy, and others, then trains Natsu in secret to become stronger and take revenge on those who wronged him.
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.
He could be betrayed by a false friend in the DxD world who spreads lies about him. For instance, telling others that Naruto has a hidden agenda to take over their realm.
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.
It could be Gray. Although they are friends most of the time, in a fanfiction, there could be a plot twist where Gray's envy of Natsu's abilities gets the better of him, and he betrays Natsu. Maybe Gray thinks that by getting rid of Natsu, he can be the top mage in their guild.