Yes, it is quite unique. Combining the wizarding world of Harry Potter with the concept of zanpakuto from Bleach is not a common occurrence in fanfiction. It brings together two very different fictional universes, which can result in some really interesting and unexpected storylines.
One possible plotline could be that Harry discovers a zanpakuto in the Forbidden Forest. He has to learn how to use it while still dealing with the threats from Voldemort. Maybe the zanpakuto has some special powers that can help him in his fight against the Dark Lord.
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.
Well, one possible plot could be that Harry discovers a Zanpakuto in the Forbidden Forest. He somehow figures out how to use it and it gives him new powers that are different from his normal magic. Maybe it allows him to access a different type of magic realm where he has to face new enemies.
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.
One possible plot could be that during one of his adventures outside of Hogwarts, Harry stumbles upon a strange object that turns out to be a zanpakuto. He has no idea what it is at first, but as he starts to explore its powers, he realizes it has a connection to a different kind of magic. He then has to learn how to use it while still dealing with the threats in the wizarding world, like Voldemort and his Death Eaters.