I think Hinata could be an ally. She has always been in love with Naruto and would not stand by while he is in danger from Dark Naruto. Additionally, Shikamaru is smart and strategic. He might see the threat of Dark Naruto to the village and decide to assist Naruto, using his intelligence to come up with battle plans.
Well, sometimes in 'naruto vs dark naruto fanfiction', a cool plotline is when Dark Naruto has some new powers that Naruto has to figure out how to counter. For example, Dark Naruto might be able to control shadows in a way that Naruto has never seen before. Or perhaps Dark Naruto has allied himself with a powerful dark force from outside the ninja world. Naruto then has to not only battle his dark self but also this new threat.
In fanfiction, the Rinnegan is often depicted as more powerful than the Sharingan in many ways. The Rinnegan has abilities like the Six Paths techniques which can be extremely overpowered. For example, it can allow the user to control gravity, summon large creatures, and even revive the dead. While the Sharingan has its own set of cool abilities like predicting movements and casting genjutsu, it seems limited in comparison when it comes to the grand scale of power shown by the Rinnegan in most fanfictions.
In a fanfiction, the aliens could capture Naruto and try to study his unique abilities. The monsters then decide to rescue him because they realize his powers could be the key to defeating the aliens once and for all. The story could involve a lot of action as the monsters infiltrate the alien base to save Naruto.
In a fanfic, Naruto might power up by having a spiritual awakening. He could connect with the spirits of past Hokages in a deeper way, gaining their knowledge and some of their power. This could enhance his existing jutsu and give him new abilities. Vegeta could power up if he were to face a near - death experience against Naruto. This could trigger a hidden potential within him, like a new type of Super Saiyan form that has unique abilities. He could also fuse with another Saiyan (if there were another Saiyan available in the fanfic's scenario) to create an even more powerful being.
Naruto would win. He has a lot of experience fighting powerful opponents and has developed many powerful jutsu over time. He also has the willpower and the ability to adapt in battle. Shukaku is strong, but Naruto's intelligence and his ability to think on his feet would give him the edge.
One common theme is self - improvement. Both Sakura and Naruto are constantly striving to get better in these fanfics. For example, Sakura might be trying to improve her combat skills to be on par with Naruto. Another theme is friendship. Despite their rivalry, their underlying friendship is often explored. It shows that they can compete and still care for each other.
I recommend "I am the Second Young Master of the Flagwood Family" to you. The main character of this novel, Flagwood Miren, transmigrated to the Naruto World and became the second young master of the Flagwood Family. He is the same age as Kakashi. The plot of this novel also included other Naruto characters, such as Haki Shuogu, Ban, and Bo Fengshui Gate. I hope you like this fairy's recommendation. Muah ~😗