The Akatsuki would surely be affected. They would lose one of their members, which could disrupt their plans. Also, it might change the way other characters view Naruto. Some might see him as a more formidable threat, while others might see him as a hero for taking out a dangerous enemy.
Well, it could happen if Naruto has some sort of special weapon. For example, he might get a sword that can disrupt Hidan's connection with his voodoo - like powers. Once that connection is broken, Naruto could easily defeat and kill Hidan. Also, Naruto's growth in chakra control and strength could play a role. With his enhanced abilities, he could overpower Hidan and deliver a fatal blow.
Well, on one hand, Hidan's immortality gives him an edge as he can keep coming back no matter how many times he's 'killed'. But Naruto is not just any opponent. Naruto has the power of the Nine - Tails and has trained with some of the greatest ninja. His speed, strength, and intelligence in battle are remarkable. He could potentially use his shadow clones to distract Hidan and then land a fatal blow. So, overall, Naruto has a greater chance of being the stronger in this fanfiction encounter.
Kakuzu would be a great character to pair with Hidan in fanfiction. They were partners in the original story and had a very interesting dynamic. Kakuzu was always exasperated with Hidan's wild and unruly ways, which could lead to some humorous and tense situations in a fanfiction.
The OC could be an outcast in the ninja world. They don't belong to any major clan and are constantly on the move. Hidan might see this OC as an easy target. The OC might have some unique skills, like being able to communicate with animals, but these skills are not enough to protect them from Hidan when he decides to sacrifice them for his own dark purposes.
It's a complex situation. Different storylines and villains have targeted the Avengers, but no one specific character can be said to have killed them all outright.