Perhaps the keyblade is from another dimension that somehow ends up in Naruto's world. Naruto, being the curious and adventurous type, starts to figure out how it works in secret. He might find that it can open portals to other places, which could lead to him exploring new lands and encountering strange creatures. And all this while he's trying to keep it hidden from the Hidden Leaf Village so as not to cause a stir.
Well, in this fanfic, Naruto might use the keyblade's power to fight new and different enemies. Maybe he discovers the keyblade during one of his solo training sessions in a hidden part of the forest. He could keep it a secret because he doesn't fully understand its power at first and doesn't want others to be afraid of him.
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.
Well, perhaps Naruto could discover the Keyblade in a hidden realm within the Naruto world. He's neglected it because no one around him knows what it is. Then he has to figure out its powers on his own, like how to use it for combat or to unlock special portals.
One possible plot could be that Naruto, who somehow gets access to a Keyblade, teams up with Aqua. They might go on a journey through different worlds, combining Naruto's ninja skills and Aqua's magic. Maybe they are on a quest to save both their home worlds from a common threat.
His confidence would likely grow. Having a powerful and unique weapon like the keyblade would make him feel more capable. He might become more independent in his decision - making as he doesn't want to risk revealing the keyblade's existence. For example, he could start taking on solo missions more often.
Maybe Naruto is sent on a special mission to a foreign land where high - tech weaponry is common. He secretly gets his hands on some of these weapons, like a wrist - mounted energy blaster. He uses it sparingly in battles back in Konoha to protect his village without revealing his new arsenal. The story could revolve around him trying to keep it a secret while also learning how to fully utilize these high - tech weapons.
One possible plot could be that they meet in secret locations like an abandoned training ground. They start sharing their deepest secrets and feelings. As they date secretly, they have to be careful not to be seen by Sasuke, who has his own complex relationship with Itachi. Their relationship could also be a way for Itachi to redeem himself in a different way, by being with Naruto who represents hope in the Naruto world.
One possible plot could be that Naruto wants to keep an eye on Sasuke after he returns to the village. He secretly moves in to make sure Sasuke is truly back for good and to support him in his reintegration. Naruto might also hope to rekindle their old friendship in this close - quarters situation.
One possibility could be that Naruto has been poisoned by a new and unknown enemy. This poison is slowly eating away at his life force, and he's trying to hide it from his friends and family so as not to worry them. He might be secretly seeking a cure while still fulfilling his duties as Hokage.
Maybe Naruto has always idolized the Hokage in secret. He could be studying the ways of past Hokages on his own, trying to learn their leadership skills and jutsu. For example, he might be secretly practicing Minato's Flying Thunder God Technique because he admires his father so much.
One possible plot could be that Naruto faked his death to go undercover in a powerful and evil organization. He pretends to be a new recruit while secretly gathering information to take the organization down from the inside.