" I can recommend One Piece: The Rabbit on White Beard's Boat." This light novel tells the story of the newcomer Mihawk meeting a rabbit with a knife when he was out at sea. The story also involved the role of Eagle Eye Mihawk. I hope you like this recommendation.
It could be a very interesting story. Harry Potter is from the world of magic, and Hawkeye Mihawk is a skilled swordsman from a different fictional universe. Maybe Hawkeye could teach Harry some unique combat skills that combine swordsmanship with magic. For example, Harry could learn to use his wand like a sword and cast spells in a more offensive and direct way during battles.
Language could be a problem. They are from different fictional universes, so communication might not be straightforward at first. Harry would have to learn new terms and concepts related to Hawkeye's training. Also, Harry might face challenges in terms of endurance. Hawkeye's training is likely to be very physically demanding, and Harry, who is more used to relying on magic for tasks, would need to build up his physical strength and stamina.
Zoro would be an obvious choice. Since Zoro has always aspired to defeat Mihawk, a crossover could show them in a situation where they have to put aside their rivalry and fight together. It could be a great opportunity to explore their relationship further and how they might learn from each other in combat.
One character that would fit well is Robin. Her knowledge of the world and its secrets could be relevant if there's some mystery involved in the Zoro - Mihawk relationship. Brook, with his unique personality and his own skills as a swordsman, could also be included. He could have some interesting conversations with Zoro and Mihawk about swordsmanship. And Shanks, as a great swordsman himself and a significant figure in the One Piece world, could potentially play a role in the story, perhaps as an old acquaintance of Mihawk.