One possibility could be that young Xehanort grabs Kairi as part of a plan to manipulate Sora. He might think that by having Kairi in his control, he can draw Sora out and into a situation where he can be easily influenced or defeated. This could lead to a series of events where the heroes have to figure out how to rescue Kairi from Xehanort's clutches while also dealing with his larger scheme.
It could be that young Xehanort grabs Kairi as a distraction. He wants to create chaos among the guardians of light. By taking Kairi, he knows that the others will be focused on finding her and rescuing her. This gives him time and opportunity to carry out other parts of his plan, like infiltrating certain key locations or stealing important items. The fanfiction could then be about how the friends balance their search for Kairi with protecting the overall world from Xehanort's other threats.
Aqua, if she were involved, would be determined to save Kairi. She has a strong sense of justice and protecting the innocent. She would likely use her magic to try and track Xehanort and Kairi. Donald and Goofy would be loyal to Sora and follow him into the fray, but also try to keep Sora from making hasty decisions. They would offer their support in combat and also try to think of practical ways to free Kairi, like finding Xehanort's weaknesses.
He might capture her for power. Kairi is an important figure, and her capture could give him an edge.
Well, it could be that Young Xehanort captures Kairi to gain some kind of power. Since Kairi has her own unique abilities and significance in the Kingdom Hearts universe. He might think that by having her in his control, he can manipulate events to his advantage. For example, he could try to use her to access certain keyblade powers or to open paths to different worlds that are otherwise inaccessible.
Kairi could be looking for an opportunity to use her physical combat skills. If she can get close enough or find a moment of weakness in Young Xehanort's grip, she might try to land a blow or use a self - defense move to break free. She may have learned some basic combat moves from training with Sora and Riku, and she could be relying on those in this situation.
One possible reason could be that Young Xehanort is very powerful. His strength might be overwhelming for Kairi, both physically and in terms of his dark magic if he uses it against her. Another reason could be that Kairi is in an unfamiliar situation. Maybe she's in a strange place or time where she has no allies to help her, which makes it difficult for her to fight back against Young Xehanort's grip.
You can often find such fanfiction on dedicated fanfiction websites. Some popular ones include Archive of Our Own. Just search for 'Young Xehanort and Eraqus' on the site, and you'll get a list of relevant stories.
A fanfiction might focus on a journey they took together. They could be on a quest for a powerful artifact. Along the way, they face various challenges that test their friendship and their understanding of the Keyblade. For example, they might encounter a group of powerful Heartless and have to combine their skills to defeat them.
A really exciting plot could involve a world where Young Xehanort and Sora's memories are swapped. They have to navigate each other's lives and relationships while also dealing with the threats in their own respective worlds. Sora would be faced with the darkness that Young Xehanort is associated with, and Young Xehanort would have to deal with Sora's friends and their positive outlook. This would be a great way to explore their characters in new and different ways.
A possible plot twist could be that Naruto turns out to be a keyblade wielder in disguise. This would completely change the dynamic as everyone in the Naruto world and Kairi would be shocked. It could also mean that Naruto has a whole different origin and mission related to the world of Kingdom Hearts.
Well, one possible plot could be that Xehanort's essence or some form of his power gets transferred into the Harry Potter world. This power could start to corrupt some of the wizarding community, and Harry, with his strong sense of right and wrong, has to trace the source of this new evil. Along the way, he discovers more about Xehanort's past and motives, and has to use his knowledge of both the wizarding world and Xehanort's methods to defeat this new threat. It would be a complex plot that would require a lot of world - building to combine the two very different fictional settings.