In fanfiction, such a betrayal could occur due to power struggles. Maybe Rias sees Naruto as a threat to her position in the Highschool DxD world. She might fear his growing power and influence, which could overshadow her own. So, in order to maintain her status, she betrays him.
The story could start with Naruto arriving in the Highschool DxD world with his godlike powers. At first, he might try to fit in, but as he discovers the underlying politics and power struggles, he feels the need to change things. His betrayal could be when he opposes the main factions. For instance, if he stops a planned battle between the devils and angels because he believes there's a better way to resolve their differences.
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.
Well, in such a fanfic, the story might develop with Naruto being betrayed in his original world and then somehow ending up in the DxD highschool setting. He could start off as an outcast due to his past betrayal, but gradually gain the trust and love of the harem members through his strong will and unique abilities.
Perhaps Naruto would try to find allies among the outcasts in the Highschool DxD world. Since he has been betrayed, he would understand the feeling of being alone and might reach out to those who are also on the fringes of society. He could form a new group that challenges the existing power structures in the Highschool DxD universe, using his unique abilities and his unwavering will from the Naruto world.
Naruto would likely become very distrustful. After being betrayed by Rias, he would find it hard to trust anyone else in the Highschool DxD world. This could isolate him and make his journey much more difficult as he would be on his own without allies.
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 idea could be that Naruto is somehow transported to the world of Highschool DxD. He could use his ninja skills and chakra in the new world, maybe even teach some of the characters there a thing or two about combat. For example, he could show Issei how to better control his power using chakra control techniques.
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.