He could have a 'Solar Sage Mode'. In this form, his body would be surrounded by a bright, sun - like aura. His physical abilities such as strength, speed, and agility would be enhanced to an extreme level. His attacks would also carry the power of the sun, like solar - powered punches or kicks that could cause massive damage.
One possibility could be the power to control time. In fanfiction, this could be used in various ways, like going back to prevent tragedies or to gain more time to train. It would give Naruto a huge advantage in battles and in his quest to protect the village.
It could be that he discovers the new power while exploring an ancient and mysterious place. There might be some kind of energy source or relic in that place that unlocks the power within him. He might have to solve some puzzles or overcome some trials to fully access the power.
This new power could potentially make Naruto more self - reflective. He has always been a bit of a hot - head, but the power of the sun is so different from his normal chakra. He would need to take time to understand this new aspect of himself. It could lead to him being more in tune with his emotions and those of others. He might start to see the world in a different light, not just as a place to protect, but also as a place to nurture with his newfound power.
Some would be in awe. The villages that Naruto has helped in the past would likely see it as another sign of his strength and a great asset. For example, the Hidden Leaf Village would feel even more protected. However, the other villages that have been enemies or rivals in the past might see it as a threat and start to plot against Naruto and the Leaf Village.
He might gain enhanced perception. This could allow him to see chakra flow more clearly, similar to how the Sharingan can. So in battles, he can anticipate his opponents' moves better.
In a fanfiction, Naruto having the power of the sun could potentially have a huge impact on the chakra system. Since the sun is such a powerful energy source, it might disrupt or enhance the existing chakra flow. He could develop new jutsu that are based on solar energy rather than just the normal chakra types. This could lead to a whole new set of skills and abilities that are unique to him, and it would force his opponents to completely re - evaluate their strategies when facing him.
In some fanfictions, Naruto's hidden power could be the ability to control a powerful and ancient force. For example, he might be able to harness the power of some long - forgotten bijuu or a mysterious energy that was sealed away in the past. This would give him abilities that are on a completely different level compared to his normal skills.
Naruto could attract the attention of other villages' ninjas who are interested in his unique power. This could lead to new alliances or rivalries. Some might want to learn from him, while others might see him as a threat. So, new diplomatic relationships could be formed based on how other villages react to his all - elemental - affinity power.