You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own. They have a large collection of Naruto - related fanfictions, and you can search specifically for ones where Naruto trains seriously.
One great 'naruto trains seriously fanfiction' could be where Naruto trains under a reclusive master. He might learn unique jutsu and develop his skills in a way different from the original series. For example, he could focus on combining nature chakra in new ways during his training.
Sakura could also be present. She might be observing the training and learning some things herself, or she could be helping Kakashi in some way, like providing medical support during intense training exercises.
In some fanfictions, Kakashi might train Naruto by creating extremely challenging survival scenarios. For example, he could drop Naruto in a mock battlefield filled with various traps and enemies that Naruto has to overcome using his skills and wits. This forces Naruto to think on his feet and improve his combat abilities quickly.
In some fanfictions, Naruto's serious training after Mizuki might lead to him rapidly improving his skills. He could master new jutsu faster, like developing a more powerful Rasengan variation. His physical strength and endurance would also increase significantly, allowing him to better handle difficult missions.
One possible plotline could be Kakashi teaching Naruto some super - advanced jutsu that Naruto then uses to surprise his friends in a sparring match. For example, Kakashi might teach Naruto a special lightning - style jutsu that Naruto masters and combines with his Rasengan.
Well, one possible plot could be that Naruto, having experienced so much growth himself, decides to pass on his unique ninja skills and values to a young Kakashi. He might teach Kakashi some of his signature jutsus like the Rasengan, but also lessons about friendship and never giving up. Naruto could also share his own experiences of dealing with loss and loneliness, which would be really interesting considering Kakashi's own past traumas.
One possible result is that Naruto becomes much stronger in a shorter period compared to the original story. His combat skills improve significantly, allowing him to take on more difficult missions. For example, he might be able to defeat enemies that were previously out of his league with ease.
He might learn some advanced chakra control techniques. For example, better management of his Nine - Tails chakra. Kakashi, being a skilled ninja, could teach Naruto how to blend his own chakra with the Nine - Tails' more effectively, which would enhance his combat abilities.
Kakashi's training style, which is often strict and methodical, would likely make Naruto more disciplined. Naruto is usually a bit of a wild card, but under Kakashi's tutelage, he would have to follow a set routine and be more precise in his actions.