One possible plot could be that Harry discovers he has Haki during his time at Hogwarts. He first realizes it when facing a Dementor and his Haki flares up, scaring the Dementor away. Then, he starts to secretly train his Haki with the help of Hagrid, who has some knowledge of strange powers. As he gets better at it, he uses it to sense the true intentions of people around him, like when he suspects Snape. Eventually, he uses his Haki in the final battle against Voldemort, giving him an unexpected edge in the fight.
Well, for a 'Harry Potter uses Haki' fanfiction, Harry could inherit the ability to use Haki from his parents in a way that was dormant until a certain event. Maybe when he gets the lightning - shaped scar, it also unlocks his Haki potential. He initially has trouble controlling it, and it causes some chaos in the Gryffindor common room. Hermione tries to research about Haki in the library but finds nothing. As time passes, Harry learns to use his Haki to communicate with magical creatures on a deeper level. When facing Voldemort, his Haki - powered magic combines with the power of love, which was always his greatest strength, and he defeats Voldemort once and for all.
In a 'Harry Potter uses Haki' fanfiction, Harry might learn about Haki from a mysterious old wizard who visits Hogwarts. This wizard senses that Harry has the potential for Haki. Harry then begins to train in both the Observation and Armament Haki. With Observation Haki, he can predict the moves of his opponents in duels, like Draco Malfoy. His Armament Haki allows him to strengthen his physical attacks. In the battle against the Death Eaters, his Haki - enhanced spells are much more powerful, and he can even break through some of the dark magic defenses of Voldemort's followers.
Dumbledore could be the one to teach Harry about Haki. Dumbledore, with his vast knowledge of magic and the unknown, might have come across Haki in his travels. He could recognize the signs of Haki in Harry and take it upon himself to guide Harry in understanding and controlling this new power. He would use his calm and wise approach, just like when he teaches Harry about other aspects of magic.
Perhaps in this fanfiction, Harry Potter is given a gun by a mysterious figure who claims it's a more powerful weapon than magic. Harry, being skeptical at first, tries it out and finds that it has some advantages in certain situations. For example, it can be used more quickly and discreetly than magic in a crowded muggle area. But this newfound weapon also makes him a target of both the dark wizards who see him as a threat and some of the good wizards who think he's gone astray. It could lead to a whole new adventure where Harry has to justify his use of the gun and find a balance between magic and this new form of weaponry.
Haki can completely shift the power dynamics. If a character like Ron suddenly discovers Haki, he could become a much more formidable opponent. He might be able to use it to intimidate Death Eaters or other dark wizards, adding an edge to his character that wasn't there before.
One possible plot could be that Voldemort discovers cybernetic technology and tries to use it to enhance his dark magic. Harry and his friends then have to learn about cybernetics quickly to counter this new threat. They might seek help from the Muggle world's experts in cybernetics.
It could be that his training in magic somehow awakens his haki. As he delves deeper into advanced magic spells and combat training at Hogwarts, his body and mind start to develop this new ability. For example, during a particularly intense dueling session, he feels a strange energy coursing through him which he later realizes is haki. His professors might initially be baffled by this new power, but as he learns to control it, it becomes an asset in his battles against the forces of evil.
Well, he might become an even more formidable wizard - warrior. The katana's swift and deadly nature could add a new dimension to his combat skills. He could use magic to enhance the katana's strikes, like making it cut through magical shields more easily.
In a 'Harry Potter uses revolvers' fanfiction, the whole tone of the story might change. The presence of a revolver in a world full of magic would introduce a whole new set of rules. For example, how would magic and bullets interact? Would Harry be able to use magic to enhance the revolver's capabilities? It could also lead to some interesting character development as Harry has to adapt to using a non - magical weapon in a magical world. This could cause conflicts with his friends who are used to the traditional magical ways of fighting.
In a fanfiction scenario where Harry Potter uses a lightsaber, his magical abilities could potentially interact with the lightsaber in interesting ways. Maybe his magic could enhance the lightsaber's power or he could use it in combination with spells for more powerful attacks. For example, he could cast a disarming spell while simultaneously using the lightsaber to keep his opponents at bay. Also, the lightsaber's unique properties could make him a more formidable opponent against Voldemort. It would change the dynamics of his duels and adventures quite a bit.
There might be a fanfiction where the concept of Haki is used to explain some of the more mysterious aspects of the wizarding world in Harry Potter. For example, how certain powerful wizards seem to have an almost instinctual knowledge of when they are in danger. It could be that they unknowingly have a form of Haki. This type of fanfiction would blend the two universes in a really unique way.
I think Conqueror's Haki would suit Naruto. He has that natural leadership and strong will, just like characters in One Piece who possess Conqueror's Haki. It would be a great addition to his existing confident and determined personality.