His friends like Sasuke and Sakura could be his allies. Sasuke might be initially skeptical but then see the potential of Naruto's arcane power in fighting common enemies. As for enemies, Orochimaru could be interested in Naruto's power for his own experiments. He might try to capture Naruto to study and steal his arcane abilities.
One plot idea could be Naruto discovers his arcane power during a mission in a hidden ancient ruin. He has to learn to control it quickly to defeat a new, powerful enemy that is immune to his normal ninja skills. Another could be that his arcane power is tied to an ancient prophecy, and he must fulfill certain tasks to fully unlock it while dealing with those who fear his newfound strength.
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.
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 challenge could be controlling the new power. Just like Naruto had to learn to control the Nine - Tails' chakra, he'd have to figure out how to use Achilles' power without overexerting himself. Another might be dealing with enemies who would target his new weakness, if he has one like Achilles' heel.
Well, in many ways. Fanfictions with Naruto having new powers can reshape the Naruto universe. They can add more depth to the existing power hierarchy. If Naruto suddenly has some overpowered ability, it can disrupt the balance between the different ninja clans and villages. This might lead to new political situations and power struggles. Also, it can affect the training and development of other characters. They might start to train in different ways to keep up with Naruto's new powers. Moreover, it can influence the overall theme of the Naruto universe. For example, if Naruto has powers related to immortality in a fanfiction, it can change the concept of life and death in the Naruto world, which is a fundamental aspect of the original story.
In such a fanfiction, Naruto might start to reshape the ninja world entirely. He could create new jutsus that are unimaginable with normal power limits. He could also travel to different dimensions effortlessly to learn more about the secrets of chakra and ninjutsu, and then bring back that knowledge to improve the lives of everyone in his own world.