Perhaps Harry makes Draco his hand. This could lead to a very different dynamic. Draco, now in this odd form, would be forced to rely on Harry. They might start to understand each other better. Harry could use this situation to try and reform Draco, showing him the good in the wizarding world that he has been blind to due to his family's influence. They could work together to solve a mystery in Hogwarts that they couldn't have solved alone.
Hermione could also be a likely character. Given how much time she spends with Harry, there are plenty of opportunities for a magical experiment or accident to occur. And with her vast knowledge, having her as a hand could be both a curse and a blessing for Harry. He would have all that knowledge at his fingertips, literally, but also have to deal with Hermione's bossy nature even in hand form.
It's possible that designating his hand is related to his connection with other characters. He could use his designated hand to make a pact or connection with a new ally or even an enemy. For instance, by placing his hand on a magical stone, he could create a bond of trust or enmity depending on the situation in the fanfiction.
Well, it could be a love letter from one character to another. Maybe Ginny secretly writes a letter to Harry, pouring out her feelings, and she hand - delivers it to him in a secluded corner of Hogwarts. This would add a romantic and secretive element to the story.
Maybe Harry discovers a new and unique magic that only he can perform. This new magic could be the mark he makes. For example, he might find a way to combine different spells in an unprecedented way that gives him an edge in fighting the dark forces.
Maybe Harry, tired of the existing power structures in the wizarding world, starts his own empire. He could start by recruiting those who have been marginalized, like werewolves and half - bloods. He'd then use his fame and skills to establish a new order, one based on equality and true understanding of magic.
Well, perhaps Harry could use magic to shape the island. He might create a magical forest filled with unique creatures from the wizarding world. He could also build a hidden school on the island to teach magic in his own way, away from the prying eyes of the Ministry of Magic.
Well, perhaps Harry Potter discovers an ancient spell in the restricted section of the library that allows him to create a golem. He might make it to protect Hogwarts from the impending dark forces. The golem could be made from the stones around the Hogwarts grounds and be enchanted with powerful defensive magic.
In this fanfiction, Harry could be forced to create an island due to some kind of dark magic threat in the wizarding world. He uses his knowledge and the help of his friends like Hermione and Ron. They might design the island to have its own unique magical ecosystem. For example, they could create a special type of tree that produces potions - ingredients as its fruits. Harry could also make a school on the island for young witches and wizards who are in danger in the regular wizarding world.
Well, perhaps the plot could start with Harry having a vision of a powerful weapon that could help in the fight against Voldemort. He discovers that Mjolnir is a weapon with great power in another mythology. He then sets out to make it. He travels to different magical places, seeking the knowledge and power required. Along the way, he has to overcome his own doubts and insecurities. He also has to deal with the wizarding world's reaction to his attempt to create a weapon from a different mythology. Some might see it as dangerous or heretical. But Harry is determined, believing that this weapon could turn the tide in the war against the Dark Lord.
One possible plot could be that Harry Potter, through some magical mishap, ends up in a world where Kusanagi exists. He might use his magic to help Kusanagi in a battle against a common enemy. For example, Harry could use spells like 'Expelliarmus' to disarm Kusanagi's foes.