One interesting plot idea could be Sora being turned into a vampire while in a new, dark world in the Kingdom Hearts universe. He might struggle with his newfound vampiric urges while still trying to fulfill his duty to protect his friends and the worlds. Another could be a story where Sora has to face off against an ancient vampire lord who is threatening the balance of the worlds. He could use his unique Keyblade skills in combination with his new vampire abilities.
One plot idea could be Sora and Xion going on a journey to discover a hidden world within the Kingdom Hearts universe. They might face unique enemies and puzzles along the way.
Perhaps they could be transported to a world where the rules of the Kingdom Hearts are reversed. Sora, who is usually the more positive and optimistic one, starts to doubt himself, while female Riku has to be the one to boost his morale. Their adventure in this topsy - turvy world could lead to some really interesting character development and new revelations about their relationship.
Dark Sora in fanfiction is really an interesting concept. Fan writers have a lot of freedom to play around with his character. For instance, they can create scenarios where Dark Sora teams up with villains in a way that wasn't shown in the main story. Or they could delve into his past and show how certain events led to his transformation into a darker version. This gives a whole new perspective on the Kingdom Hearts world and the character of Sora as a whole.
Donald and Goofy could still be his allies. Despite Sora being a vampire, they might see that he's still the same Sora at heart. Riku could also be an ally, as he has always been there for Sora in different situations. Maybe even some of the Disney characters, like Mickey Mouse, who is known for his kindness and understanding, would be willing to help Sora control his vampiric side and fight against any threats that come with it.
One idea could be exploring the backstories of the new characters introduced in Kingdom Hearts 3 in more depth. For example, you could write about how they came to be in the different worlds and what their true motives are.
Well, it depends on how you look at it. Technically, in official Kingdom Hearts content, Sora is a key character. But in the realm of fanfiction, many writers take the essence of Sora and create their own unique stories that are not part of the official canon but are still inspired by the Kingdom Hearts universe. So, in that sense, he can be considered as a major element in Kingdom Hearts fanfiction.
Some fanfictions might have a storyline where Sora and Kairi finally get a chance to have a normal life for a while. They could be in a small, peaceful world where they go on dates, learn about each other's dreams, and just enjoy the simple things. But of course, trouble always finds a way to disrupt their peace.
Perhaps Sora's keyblade has some strange reaction in the Dragon Maid world. It could start to glow differently or even change shape when he encounters certain magical elements in this new world. He then has to figure out why this is happening while also getting used to the unique customs and characters in the Dragon Maid world, like Kobayashi's daily life with the dragons.
One idea could be that the Heartless somehow find their way into the Marvel universe and start attacking various locations. The Avengers, being the protectors, team up with Sora and his friends from Kingdom Hearts. They initially have some clashes due to different fighting styles and ways of understanding the world, but eventually learn from each other and combine their powers to defeat the Heartless. For example, Iron Man could use his technology to analyze the Heartless' weaknesses, while Sora uses his Keyblade to directly combat them.
One idea could be that Sora finds himself in a new world where he is surrounded by characters from different Kingdom Hearts realms who all have a strong attraction to him. They compete in a friendly way to go on adventures with him.