He could practice in secret locations where no one can see him. For example, in an abandoned cave or a hidden part of the forest. That way, no one will know about the new jutsu he's developing.
Perhaps Naruto hides his jutsu because he's being spied on by an unknown enemy within the village. He doesn't want to expose his new and powerful techniques until he can figure out who the spy is. So he acts like he's still relying on his old moves during training and small skirmishes.
In fanfic, Naruto may create his own jutsu based on his emotions. His strong will and never - give - up attitude can be translated into chakra - based techniques. For instance, when he's feeling extremely protective of someone, he channels that emotion into his chakra and creates a defensive jutsu. He also might draw inspiration from his memories. Remembering his training sessions with Jiraiya, he could take some of the basic techniques he learned and add his own twist to them, like adding a wind - element boost to a simple physical attack jutsu.
Another reason could be related to his personal growth. He might be trying to master the power of the eye in private. Naruto has always been one to face challenges alone at first. By hiding his eye, he can focus on understanding and using its power without distractions or the pressure of others' expectations.
One key factor is his vast combat experience. After facing various enemies, he knows the weaknesses and strengths of different jutsus, which helps him create his own. Another factor is his ability to learn from others. He has trained with many great ninjas like Kakashi and Jiraiya, and he can pick up on their techniques and make them his own. Also, his strong bond with the Nine - Tails gives him access to a large amount of chakra and unique chakra properties that he can use in creating new jutsus.
Perhaps Naruto is waiting for the right moment to reveal his skills for maximum impact. He could be planning a big surprise for his enemies or wants to make a grand entrance at a crucial time. For example, in a major battle against a powerful foe, he could suddenly unleash all his hidden skills and turn the tide of the battle, which would be more effective than showing them off gradually.
Maybe he wants to protect his loved ones. If his enemies know his true strength, they might target his friends and family to get to him. So, by hiding his skills, he can keep them out of harm's way while still being able to fight when necessary.
One consequence could be that his friends might start to suspect him. Since Naruto is usually an open book, hiding something as significant as his eye could make them wonder what he's up to.
Perhaps he has hidden the full extent of his Sage Mode abilities. He might be able to stay in Sage Mode for a much longer time than he has shown so far, or have some other undisclosed powers within that mode, like the ability to sense enemies from much greater distances.
Maybe he found some hidden scrolls left by his mother. These scrolls could have contained the details of her jutsu. Naruto, being the curious and determined ninja he is, would have studied these scrolls carefully and learned the jutsu.