In 'naruto haki fanfic', Haki can change the Naruto universe by adding a new layer of power. For example, it can make battles more complex as characters now have to consider both chakra and Haki. It might also change the way characters interact with each other, as those with Haki could sense things differently.
Percy often uses Observation Haki to anticipate the attacks of his enemies. He can sense their movements and intentions, which gives him an edge in combat, especially against fast - moving or sneaky foes like some of the monsters in the Percy Jackson world.
Perhaps Naruto discovered a hidden power within himself that was similar to Busoshoku Haki. In the fanfic, the author might have created a new backstory for Naruto where his Uzumaki heritage had some latent abilities that could be manifested as Haki - like powers. So, he just had to figure out how to use it on his swords.
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.
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.
An OC's Haki can be developed through intense training. Just like how Luffy and the others train in the One Piece world. They could face difficult opponents and learn from each battle, gradually strengthening their Haki. For example, if they constantly fight against Logia - type devil fruit users, their Armament Haki would improve to counter their intangible forms.
In some fanfictions, Luffy's Haki training often involves extreme challenges. For example, he might face powerful foes that force him to push his Haki to new limits. He could be in situations where his Observation Haki has to be sharpened quickly to avoid deadly attacks. Sometimes, he trains with new mentors in fanfic worlds who have unique ways of teaching Haki, like making him endure intense mental and physical stress to strengthen his Conqueror's Haki.
In Naruto fanfiction with Haoshoku Haki, it could be incorporated in battles. For example, a powerful Naruto might unknowingly use a form of Haoshoku Haki during a crucial fight against a strong enemy. This could be shown as his intense willpower overwhelming his foes in a way similar to how it works in One Piece. It might cause some enemies to faint or lose their fighting spirit.