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.
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.
Naruto would likely be deeply hurt. He is a character who values friendship and loyalty highly. Being betrayed by someone he might have considered an ally like Rias would be a huge blow to his emotional state.
He might react with hostility. Dark Naruto has a certain edge to him, and if he sees Rias as a threat or someone he doesn't like, he'll likely oppose her. He could use his combat skills to challenge her directly.
In a Highschool DxD fanfiction, if Naruto kills Riser, it would be a major event. First of all, the devils in the story would be in an uproar. Riser was an important figure among them. His death could lead to a power struggle among the devils to fill the void he left. Naruto would then have to face not only the devils who want revenge but also the potential chaos that his action has caused. For example, the Rating Games system might be affected as Riser was a key player in it. There could also be a shift in the relationships between different species in the Highschool DxD world, like the relationship between devils and angels might change depending on how they view Naruto's action.
Well, it would be quite an interesting relationship. Naruto's strong will and combat skills from his own world could mesh well with Rias' leadership in the Highschool DxD universe. He would likely bring a new sense of loyalty and determination to her peerage. Also, his ability to make friends easily might help him fit into the complex social structure of the DxD world.
Well, in some fanfictions that depict Rias as a spoiled brat, they might show her using her status and power to get what she wants in the Highschool DxD universe. For example, she could be overly demanding with her peerage members.
In this kind of fanfiction, Naruto might face a lot of challenges. Maybe Rias, for some reason, turns against Naruto. Naruto could find himself isolated and having to fight against Rias' forces. He would probably rely on his ninja skills and his strong will to survive.
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 great fanfiction idea could be that Issei discovers a new power within himself that is directly linked to Rias's heritage. Maybe he can tap into an ancient magic that only works in harmony with Rias's own powers. As they train together to master this new ability, their relationship deepens beyond just the typical master - servant bond in the Highschool DxD world.
A common theme is power - up. Rias may gain new powers or improve her existing ones. This could be related to her devil heritage or through some special training or encounter.