Well, perhaps Harry Potter discovers some ancient magic related to the Jagan in this fanfiction. Maybe he has to face new and powerful enemies as a result of awakening it.
In this fanfiction, it could be that the appearance of the keyblade is a sign of a greater prophecy. Harry, being the chosen one in his own world, is now also the one to awaken the keyblade. He has to face challenges like learning new combat styles, understanding the keyblade's magic which is different from his usual spells. And along the way, he might make new friends or enemies related to the keyblade's origin and purpose.
Well, he could start by trying to find a way back to his own world. But in the process, he gets involved in the affairs of Spira. He might meet some characters who are similar to his friends at Hogwarts. They could form a group and together they explore the mysteries of Spira. Harry could also share his knowledge of magic with the local wizards (if there are any), and in return, learn about the unique magic system of Spira.
In addition to Harry, Hermione, and Ron, there could be some new characters introduced specifically related to the Jagan. Maybe an old wizard or witch who has some knowledge about the Jagan in the past. They could either guide Harry or try to stop him from getting too involved with it. And of course, there may be some villains who sense the power of the Jagan awakening and want to take advantage of it for their evil plans. Dumbledore or other Hogwarts professors might also play important roles as they often help Harry in difficult situations.
A new character from the Kingdom Hearts universe could be the main antagonist. Maybe a powerful Heartless or a dark Keyblade wielder who sees Harry as a threat since he has awakened the keyblade. This antagonist could be after Harry to take the keyblade for their own evil plans.
Well, perhaps in this fanfiction, the devil gene is awakened when Harry is exposed to a powerful and cursed object. As the gene awakens, he starts to have strange visions and an uncontrollable urge for power. His Hogwarts experience completely changes as he has to deal with this new aspect of himself. He may also attract the attention of dark forces that want to use his newfound power for their own evil purposes.
Another theme is the cultural exchange. The characters from the two universes come from very different backgrounds. So, in the fanfiction, you might see how a female Harry Potter adapts to the Star Wars universe and vice versa. This could include things like how she reacts to the different social hierarchies or the technology in the Star Wars world.
One possible plot could be that Harry Potter, while exploring new magical realms, stumbles upon a portal that leads him to the Star Wars universe. There, he meets Rey and they discover that magic and the Force have some similarities. They could team up to fight against a new dark force that is threatening both their worlds.
Obviously, Harry Potter and Rey could be the main characters. Harry has his skills in magic and combat against dark forces, and Rey has her connection to the Force and her combat skills. They could be the central duo, with each learning from the other.
It could be Voldemort still. Since Harry has awakened the devil gene, Voldemort might see this as an opportunity to either corrupt Harry further or use his new power for his own dark purposes. So he would be the main force against Harry.