Harry Potter himself could be the main character. Imagine him stumbling upon a dark keyblade and having to grapple with its power. He would have to decide whether to use it to fight Voldemort or risk being corrupted by its dark influence.
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.
One interesting concept could be the fusion of the magical world of Harry Potter with the Keyblade concept from Kingdom Hearts. It might involve creating new spells or powers related to the Keyblade within the Hogwarts setting. For example, a character could use a Keyblade to unlock hidden chambers or secrets in the castle that are otherwise inaccessible. Another concept could be the idea of a dark Keyblade wielder in the Harry Potter universe who uses the power for evil, perhaps to take over the Ministry of Magic or to control the Dementors.
It could be an original character created specifically for this fanfiction. Maybe a young wizard or witch who has always been an outcast in the wizarding world and discovers their unique ability to connect with dark dragons.
Yes, there are likely to be such fanfictions. The 'Harry Potter' and 'Keyblade' universes are both rich in their own elements, and fans often like to combine different fictional worlds in fanfiction. You can try to search for it on major fanfiction websites like FanFiction.net or Archive of Our Own.
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.
In fanfiction, the combination of Harry Potter elements with an Aqua Keyblade and Fire can create really interesting scenarios. It could be a story where Harry discovers a magical Keyblade that has the power of water (aqua) and is able to control fire as well. This might lead to new adventures and challenges for him, like facing off against dark wizards who are also after the power of this unique Keyblade.
Perhaps an old Hufflepuff alumni who has turned to the dark side. This person was once a great student but became corrupted over time. They return to Hogwarts with the intention of using the Hufflepuff house as a base for their dark plans. They manipulate the Hufflepuff students, taking advantage of their kindness and hard - working nature, and turn it into a means to an end for their evil deeds. Harry gets caught up in this as he tries to figure out who this mysterious villain is and stop their plans.
It could be an original character. Maybe a young wizard who discovers a long - lost time - travel spell. This character could be from a family that has been affected by the dark arts in the past and travels back to change their family's fate. They would then get caught up in the larger events of the wizarding world during that time.
The Dark Lady could be an ancient witch who was once a powerful figure but was banished for her dark practices. Maybe she was a rival of one of the great wizards in the past and now she's back to seek revenge.
It could be an original character created by the fanfiction writer. A witch who has been in the shadows for a long time, biding her time to rise to power.