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.
Another possibility is that Rias is being controlled or influenced by some external force. In the complex world of Highschool DxD fanfiction, there could be dark magic or some other entity forcing her to act against Naruto. This external factor manipulates her actions, leading to the betrayal.
Well, in such a fanfiction, Dark Naruto might use his powerful and often shadowy abilities to outwit Rias. Maybe he'd expose some of her less - than - noble actions in the Highschool DxD world. He could easily overpower her in battles, using his ninja skills and the dark powers he has. For example, his shadow clones could be used to confuse Rias and her peerage while he strikes from the shadows.
In this kind of fanfiction, it might happen because of a misunderstanding. Maybe Rias is misled by false information about Naruto. For example, some villains could fabricate lies about Naruto's intentions or actions, causing Rias to believe he's a threat and thus betray him.
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.
A possible way for their worlds to merge in fanfiction is through a reincarnation plot. What if some of the Naruto characters were reincarnated in the Highschool DxD world? Naruto himself could be reborn as a devil or an angel. This would completely change the power balance and relationships in the Highschool DxD world. Rias would have to deal with this new Naruto, who still has his ninja spirit but now has to adapt to the rules and powers of the new world.
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.
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.
Well, it could involve Naruto characters being thrown into the Highschool DxD universe and having some sort of conflict with Rias. Maybe Naruto's skills and values clash with Rias's actions or leadership in the story. For example, Naruto might see her as too power - hungry or not treating her underlings fairly.
A really interesting fanfiction could be where Naruto's Nine - Tails chakra has an unexpected reaction when he is near Moka. Maybe it flares up in a way that he can't control at first, and Moka's vampire instincts sense the power. This could lead to a situation where they have to work together to figure out how to control this new interaction between their powers. Also, Naruto could introduce the concept of ninja teams to the characters in Highschool DxD, which could change the way they approach battles and group activities.
Well, in such a fanfiction, Naruto might feel out of place in the Highschool DxD world. He could be heartbroken because he misses his friends from Konoha. Maybe he has a hard time adjusting to the different rules and powers in the new world.