In a fanfic where Naruto uses haki, it could change his relationships with other characters too. For example, if he shows this new power, some characters might be in awe of him, while others might be jealous. His training to master haki could also lead to new interactions with mentors or allies who are familiar with this power. Also, his use of haki could potentially attract the attention of new enemies who are interested in this unique ability.
Well, there are 'luffy haki fanfic' stories that focus on the development of his Observation Haki. In some of them, Luffy is able to predict the moves of his opponents far in advance, like being able to dodge attacks that are seemingly impossible to avoid. These fanfics often add new elements to the use of his Haki, like being able to sense the emotions behind an opponent's attack as well.
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.
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.
In this kind of fanfiction, usually the plot might involve Luffy starting to discover his latent conqueror's haki power. Maybe he first experiences some intense battles where he unknowingly shows signs of it. Then, he might meet an old and wise character who recognizes this power in him and starts to guide him on how to control and use it effectively.
You can start by looking on fanfic websites like Archive of Our Own (AO3) or FanFiction.net. Just type 'luffy haki fanfic' into their search bars, and a whole bunch of stories should come up. These platforms have a large collection of One Piece - related fanfics, including those focused on Luffy's Haki.
If Luffy knew Haki early, it would likely give him a significant advantage. He could take on stronger opponents sooner and perhaps even prevent some of the setbacks he faced originally. This could lead to a quicker rise to power and a different dynamic with his crew.
Aside from Luffy, characters like Ace might show up in this type of fanfiction. Ace, being Luffy's brother, could have some connection to Luffy's discovery of conquerer's haki, perhaps through memories or some form of spiritual guidance. Additionally, some powerful enemies might be present. These enemies would serve as the motivation for Luffy to quickly master the conqueror's haki in order to defeat them and protect his crew. There may also be some minor characters from the One Piece world who assist Luffy in his learning process, like the inhabitants of a special village that has knowledge about haki.
In some fanfictions, Naruto might discover haki during a particularly intense battle. Maybe when facing an enemy with an unusual power that his regular jutsu can't handle, he unknowingly taps into his latent haki ability.
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.