Maybe through some kind of magical encounter. For instance, he could stumble upon a portal that leads to a world where the Keyblade exists and be chosen by it. It could be that his strong will and desire to protect his friends are what draw the Keyblade to him.
In fanfiction, Naruto could obtain the Keyblade if he was in a situation where he had to prove himself worthy. He might have to go through a series of trials or battles, showing his courage, loyalty, and strength. His ninja training and experiences would give him an edge in these challenges. Once he passed all the tests, the Keyblade would choose him as its wielder.
If Naruto were a Keyblade wielder, his physical strength and chakra control could enhance his Keyblade combat. His Shadow Clone Jutsu could be used in creative ways, like sending clones to distract enemies while he attacks with the Keyblade. Also, his determination, which is a big part of his character, would make him a formidable Keyblade wielder, never giving up in battles against the Heartless or other foes.
Well, perhaps Naruto discovers a strange new power that is related to the keyblade. He could be on a mission and stumble upon an ancient artifact that somehow transforms into a keyblade. Then he has to learn how to use it while still relying on his ninja skills. Maybe he uses it to fight against a new, powerful enemy that is immune to normal ninja attacks.
One possible scenario in a Naruto Keyblade fanfiction could be Naruto obtaining a Keyblade. He might use it in a unique way, combining his ninja skills with the power of the Keyblade. For example, he could infuse his chakra into the Keyblade attacks, creating powerful and unexpected combos.
He could obtain it through a strange twist of fate. Maybe during a mission in a dimension where the Harry Potter world and the Naruto world collide, he stumbles upon it. It could be lying in an ancient tomb, and Naruto, being curious, picks it up and unknowingly becomes its master.
In a fanfiction, Naruto might obtain the Rinnegan and Sharingan through a unique encounter. For example, he could stumble upon a dying Uchiha who transfers his Sharingan to Naruto in a moment of desperation. As for the Rinnegan, it could be related to his connection with the Sage of Six Paths. Perhaps the Sage's spirit, seeing the great need in the ninja world, bestows the Rinnegan upon Naruto. It could also be a result of some sort of genetic experimentation gone wrong but in a way that gives him these powerful eyes instead of harming him.
Another way is through a series of trials. There could be some sort of magical or ninja - related challenges set up. If Naruto passes all of them, he earns the right to have the Blades of Chaos. These trials could test his strength, intelligence, and his ability to make the right choices in difficult situations.
One way could be through some kind of ancient ninja art or scroll. Maybe Naruto discovers a long - lost scroll that details the creation and use of a Reality Marble. Another possibility is that he is bestowed with it by a powerful, mysterious entity. For example, a sage from an unknown land could see Naruto's potential and give him the Reality Marble as a means to protect the ninja world.
Well, Naruto might use the Keyblade to gain power and respect. He could start training with it and show those who neglected him what he's capable of.
It could be Orochimaru. He might have heard about the keyblade and wants it for his own evil purposes. He could try to steal it from Naruto, believing that with the keyblade's power, he could become invincible. Orochimaru is always looking for new sources of power, and the keyblade would be a very tempting addition to his arsenal.
Maybe Orochimaru. He could sense the power of the Keyblade and wants to take it for himself. He would see Naruto as an easy target to steal it from since Naruto doesn't really understand its value at first.