Well, it could be really interesting. Naruto's ninja training techniques might be quite different from what Barry is used to. Maybe Naruto would teach Barry how to use chakra in some form, like to enhance his speed even more. And Barry could show Naruto some of his cool Flash moves in return.
In a Naruto trains Hanabi fanfic, the plot could be centered on Naruto introducing Hanabi to new elements of ninja life. Since Naruto has interacted with various ninja from different villages, he could expose Hanabi to different fighting styles and strategies. He might take her on a journey to different training grounds, where she can learn from the environment and face new challenges. This could also lead to Hanabi developing new friendships and alliances, all while becoming a stronger ninja under Naruto's guidance.
One possible plot could be that Naruto and Gohan start training together. Naruto is amazed by Gohan's Saiyan power and Gohan is intrigued by Naruto's ninja skills. They exchange techniques, like Naruto showing Gohan how to use shadow clones for training efficiency and Gohan teaching Naruto how to sense and control his ki. Eventually, they team up to face a common enemy that threatens both their worlds.
One possible plot could be that Naruto, having become much stronger and more experienced over the years, takes Sasuke under his wing to help him master new jutsu. Maybe Naruto teaches Sasuke some of his unique Rasengan variations, and they train in a secluded area away from the village. Along the way, they also have heart - to - heart conversations about their past and how they can better protect the village together.
Well, in such a fanfic, Naruto might gain a whole new perspective on time and space from Barry Allen. Since Barry can manipulate time to an extent, Naruto could learn to anticipate his opponents' moves even better. Also, Naruto's chakra control could be improved in ways we can't even imagine. He might be able to use his chakra in a more efficient and powerful way, similar to how Barry uses the Speed Force. And the story could involve Naruto using his new - found abilities to protect the Hidden Leaf Village from threats that are beyond the normal ninja capabilities.
One possible plot twist could be that during their training, they accidentally discover a new and powerful jutsu. Maybe it's a jutsu that combines their unique abilities. Naruto's chakra control could be enhanced by Hinata's gentle fist style in an unexpected way, leading to a whole new form of attack or defense.
The plot could involve a group of underdog ninjas forming a crazy team. They might start with really wacky and unorthodox training methods, like training on a constantly shifting terrain made by a jutsu. Their goal could be to master a unique and powerful ninja skill that no one has ever seen before in the Naruto universe.
It could be about Barry using his singing to defeat a villain. Maybe the villain is vulnerable to certain frequencies that Barry can produce with his singing voice.
One possible plot could be Barry Allen using his super speed to solve a complex mystery in Central City. He might discover a new villain who has the ability to manipulate time, and he has to race against the clock (literally) to stop the villain from changing the timeline in a disastrous way.
In a Naruto Keyblade fanfic, the Keyblade could be integrated into the Naruto universe in an interesting way. For example, the Keyblade could be related to some ancient and forgotten Ninja art. Naruto, as the main character, could be the only one able to unlock the true potential of the Keyblade. This could create a whole new power dynamic in the series. He could use the Keyblade to open portals to other dimensions, bringing in new allies or enemies. And his friends like Sasuke and Sakura could also be involved, either by being amazed at Naruto's new power or by getting their own Keyblades in some way.
The Hokage, like Tsunade or Kakashi, could be the ones training Naruto. They know Naruto well and have a lot of knowledge and skills to pass on.