One idea could be exploring Naruto's growth not just in terms of power from the Kyuubi, but also his emotional and mental growth. For example, how he overcomes the stigma of being the Kyuubi's host among the villagers who once feared and hated him. This could involve his relationships with characters like Iruka, who was one of the first to see Naruto as more than just a container for the Kyuubi.
Well, one reason could be that the author wants to explore an alternate universe. Maybe in this fanfiction's version of the Naruto world, the events leading to the sealing of the Kyuubi never occurred. It could be due to different political situations among the ninja villages, or perhaps a different chain of events that prevented the Kyuubi's attack on Konoha in the first place. So, without the attack, there was no need for the seal.
Perhaps a plot twist where the seal on the Kyuubi was never complete. And as Naruto grows stronger, parts of the Kyuubi's true form start to leak out in unexpected ways. This could lead to Naruto having to struggle not only with controlling the Kyuubi's power but also with hiding this new development from his friends and enemies alike, adding a whole new layer of tension to the story.
One possible consequence is that the power balance in the ninja world is completely disrupted. With the Kyuubi's massive chakra and power running free, smaller villages might be constantly in fear of being attacked. Konoha, in particular, would have to find new ways to defend itself without relying on the fact that the Kyuubi is sealed within one of its shinobi.
You can start by looking on fanfiction websites like Archive of Our Own or FanFiction.net. Just type in 'sealed Naruto RWBY' in the search bar and a bunch of stories should come up.
Yes, in some fanfictions, Naruto is often depicted as the Kyuubi or having a very close connection to it. This can lead to really interesting and unique storylines that deviate from the original plot.
In many such fanfictions, it could be a variety of deities. It might be an ancient and powerful god from Naruto's world's mythology that the story's author has chosen to insert. For example, it could be a god of elements like a super - powerful fire god, which could give Naruto even more control over his existing fire - based jutsu and new, more extreme abilities related to fire.
In some fanfictions, Kushina was sealed into Naruto through a complex jutsu. It might be a modified version of the sealing jutsu used to seal the Nine - Tails. The process could involve special incantations and chakra manipulations.
One common plotline is the Kyuubi teaching Naruto new and powerful jutsu. Since the Kyuubi has a vast amount of chakra and knowledge, it could share unique techniques with Naruto that are not part of the regular ninja training. Another plotline could be the Kyuubi's influence on Naruto's relationships. For example, some fanfictions explore how Naruto's connection with the Kyuubi affects how his friends and villagers perceive him. It can also lead to some interesting character development as Naruto has to learn to control the Kyuubi's power without being consumed by it.
In many Naruto fanfictions, Naruto might have developed some new and unique abilities that the Kyuubi has no defense against. This could make the Kyuubi, which is used to being a dominant and feared force, feel vulnerable. For example, if Naruto has the power to nullify the Kyuubi's chakra or to manipulate it in a way that goes against the Kyuubi's will, it would be a cause for fear.