It could be an ancient inheritance. Maybe Naruto is a descendant of a long - forgotten clan that had special soul powers, and these powers have been dormant in his bloodline until now.
The soul power could be related to the Nine - Tails. Since the Nine - Tails is such a powerful entity within Naruto, his soul power might be a way to better control or even transform the Nine - Tails' power in a more positive and spiritual way. For example, instead of just using the Nine - Tails' chakra for brute strength, he could use his soul power to make the Nine - Tails' power more harmonious and less destructive.
In Naruto fanfiction, one common power - up is related to new jutsu creation. For example, some fanfictions might have Naruto develop a jutsu that combines the power of all five chakra natures in a completely unique way. This could give him an edge over his enemies like never before. Another aspect could be the enhancement of his Sage Mode. Maybe he finds a way to make it more stable and powerful, allowing him to stay in the mode for a much longer time without any negative side effects. There are also fanfictions where Naruto gains the ability to control time, which is a completely new and overpowered concept that makes for really interesting stories.
In certain fanfic concepts, Naruto could get the phoenix power during a near - death experience. His strong will to survive and protect his friends could attract the phoenix spirit. The phoenix, known for its rebirth and powerful abilities, might choose Naruto as its host, seeing his indomitable spirit as a worthy vessel for its power.
Well, it could be that Naruto's strong will and his connection to all the other tailed beasts play a role. Since he has befriended the other tailed beasts, their combined energy or some sort of spiritual link might attract the power of the Juubi. For example, in some fanfic concepts, the tailed beasts' collective consciousness decides to merge their powers with Naruto, which includes the Juubi's power as the ultimate form of chakra.
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.
Well, it could mean that Naruto's actions become more mechanical. Without a soul, he might lack the emotional depth that drives his character in the original series. For example, his bonds with friends like Sasuke and Sakura would be very different, perhaps more of a logical alliance rather than the deep, soul - connected friendships we see in the original.
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.
In a 'naruto has no soul' fanfiction, the plot would deviate significantly from the original. His growth as a ninja would be stunted in a way. His ability to learn new jutsu might be hampered because the understanding and connection that comes with having a soul are missing. His interactions with the Nine - Tailed Fox would also change. The fox might see him as a more empty vessel and either try to take over more easily or completely ignore him as he lacks the spiritual essence it used to battle against.