In a Naruto and Rias arranged marriage fanfiction, the story might begin with a prophecy or an ancient agreement that Naruto and Rias are to be married. Naruto, who is always up for a new adventure, would approach this situation with an open mind. Rias, being a bit more cautious, would be more reserved. As they interact, Naruto's positive attitude would start to break down Rias's walls. They could go on adventures together, facing demons and other threats in Rias's world. Along the way, they would learn about each other's pasts and traumas. Naruto's experience with loneliness due to being an outcast in his village could resonate with Rias's own feelings of being different. This shared understanding could be the foundation for their relationship to develop within the context of the arranged marriage.
One possible plot could be that Naruto, being from his own ninja world, is suddenly thrust into a situation where an arranged marriage to Rias is proposed. At first, they might be hesitant and even a bit hostile towards each other. Naruto, used to his own way of life and having his own goals in the ninja world, might not understand the concept of this arranged marriage in Rias's world. However, as they start to spend time together, they could discover each other's strengths. Naruto's determination and loyalty could impress Rias, and Rias's leadership skills and magical abilities could intrigue Naruto. Eventually, they could grow to respect and even fall in love with each other, and together face the challenges that come with their union, like dealing with those who oppose the marriage from both of their worlds.