Another plotline could be that Naruto is already in a version of the Highschool DxD world that has been corrupted by Rias's rule. He then embarks on a journey to find the source of her power and a way to break it. Along the way, he meets various characters who either oppose or support Rias. Naruto has to use his wits and his ninja abilities to outsmart Rias and her minions while also trying to win over the hearts of the people who have been under her influence.
Perhaps Naruto is sent to the Highschool DxD world as part of a mission or some sort of magical accident. He discovers that Rias is using her position to take advantage of weaker characters. Naruto, being the hero he is, decides to put a stop to it. He challenges Rias to a series of duels or competitions where the loser has to change their ways. This would create a lot of tension and drama in the story.
One possible plotline could be that Naruto is transported to the Highschool DxD world and immediately notices Rias's overbearing attitude. He decides to oppose her and starts to gather allies among the other students and characters who are also fed up with her ways. As a result, a power struggle ensues between Naruto and Rias.