Naruto may be forced to fight in battles or do tasks that he doesn't want to do. His skills and powers could be misused by Rias for her own purposes. Also, his relationships with his friends in his own world might be affected as he is enslaved in another world. He may not be able to return to his home or communicate with his friends, which could cause a lot of emotional distress for him.
Well, it could be due to some powerful magic or trickery by Rias. Maybe she has some ulterior motives and uses her special abilities to enslave Naruto. But this is just speculation as it depends on how the fanfiction writer has crafted the story.
Naruto would likely be very angry at first. He is a free - spirited person who values his freedom greatly. So being enslaved would go against everything he stands for. He'd probably try to break free immediately, using his brute strength and any ninja techniques he can muster.
One possible plot development could be Naruto initially fighting against the enslavement. He might use his ninja skills and chakra to resist the control of Rias Peerage. However, as the story progresses, he could start to understand their motives and form an unlikely alliance. For example, they might be trying to protect a greater good that Naruto didn't initially see.
They would likely start with some mutual curiosity. Naruto would be intrigued by Rias's devilish appearance and powers, while Rias would be interested in Naruto's ninja abilities. They might exchange stories about their respective worlds, with Naruto talking about his village and his friends, and Rias sharing about the devil society. Interaction could also involve sparring sessions to test each other's strength.
One possible fanfiction idea could be that Rias' sick brother is a key factor in a power struggle within their supernatural world. Naruto, being an outsider with his own set of skills and values, could get involved. Maybe he forms an alliance with Rias to protect her brother from those who would use his illness to their advantage.
Definitely. The concept of combining different fictional universes in fanfiction is quite common. In a 'Naruto is Rias' Familiar' fanfiction, we could expect to see Naruto's ninja skills and powers being incorporated into the world of High School DxD. Maybe Naruto would use his shadow clone jutsu in battles in the DxD universe or his rasengan against some of the devils. It could also explore how Naruto, with his strong sense of justice and loyalty, would interact with Rias and the other characters in her peerage.
In this fanfiction, Naruto might show his jealousy in his typical brash way. He could be all fired up and ready to challenge the person who is making him jealous. Rias, on the other hand, would be more calculating in her jealousy. She might use her devilish powers and influence to subtly drive a wedge between the person causing the jealousy and the object of her affection. They could end up teaming up to get rid of the 'competition' in a rather comical or intense way depending on the tone of the fanfiction.
It's quite possible. Fanfiction writers often come up with interesting and unexpected concepts. Since Naruto and the world of Rias have their own distinct fan bases, combining them in a story where Naruto is a fan of Rias can add a new and exciting twist. You could also check fan - run forums dedicated to either Naruto or High School DxD to see if anyone can point you in the direction of such a fanfiction.
One possible reason could be that the storylines in the fanfiction are focused on different aspects or relationships within the fanfic's universe. Maybe the author wants to explore other characters' interactions more deeply and Naruto just doesn't fit into those particular plotlines they have in mind.