It could be the main character Naruto in some fanfics. Since he already has a connection with the Nine - Tailed Fox, a fanfic writer might expand on that and give him an even more powerful 'Power of the Beast' that combines the Nine - Tailed Fox's power with other beast - like elements.
Well, in Naruto fanfic, the 'Power of the Beast' may be a newly - created power concept. Maybe it's a power that combines the strength, speed, and instincts of different beasts. For example, it could be a power that allows a character to have the agility of a cat - like beast, the strength of a bear - like beast, and the senses of a wolf - like beast all at once. It could also be a power that is awakened through some special ritual or training related to beasts.
If Naruto has the power of chaos in a fanfic, it would likely change his relationships. His friends might be both in awe and a bit scared of his new power. He could use this power to break free from some of the traditional ninja constraints. Maybe he could create new jutsus that are based on chaos, like a chaotic shadow clone technique that doesn't follow the normal rules of clone behavior.
Orochimaru is a very likely character. He has always been shown to be extremely power - hungry in the original Naruto series. He's constantly seeking new ways to gain more power, whether it's through experimenting on others or trying to obtain the Sharingan. Another one could be Kabuto. He was influenced by Orochimaru and also had a strong desire for power. He tried to absorb all kinds of powers and knowledge to become stronger. And Madara Uchiha is also a candidate. His plan to create the Infinite Tsukuyomi was mainly driven by his thirst for power and his desire to reshape the world according to his own will.
Naruto is obviously a main character as the story revolves around his sealed power. Sasuke is also often included as his rival and friend, who may play a role in helping or hindering Naruto's journey regarding the sealed power.
Sasuke is a very likely character. He already has a strong connection to hate in the original series due to his clan's massacre. In a fanfic about the power of hate, his journey could be further explored, like how he might be tempted by even darker forms of hate - driven power. Another one is Obito. His fall into darkness was also driven by hate, and in a fanfic, his character could be used to show the extreme lengths that the power of hate can take someone.
Sasuke is often an interesting character in these fanfics. Since he's constantly seeking power, in a power - up fanfic, he might develop some really cool new abilities. He could gain the ability to control a new type of chakra, like some kind of dark chakra that gives him extra - powerful attacks.
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.