Well, perhaps the voices Naruto hears are actually the suppressed feelings and thoughts of the Nine - Tails within him. It could be that as he grows stronger, the seal that keeps the Nine - Tails in check weakens a bit, and he starts hearing its voice more clearly. This could lead to a story where Naruto has to learn to co - exist with these voices and even draw power from them in a more controlled way.
In this kind of fanfiction, the plot could be that Naruto starts hearing voices due to some dark jutsu used against him. Maybe these voices are from his past enemies, like Orochimaru, whispering in his mind, trying to break his spirit. And Naruto has to struggle to overcome these voices and not let them influence his actions.
It could be the Sage of Six Paths. Since Naruto has the power of the Nine - Tails and is on a path to becoming a great ninja, the Sage might be trying to communicate with him through voices, guiding him to fulfill some greater destiny.
Well, it could be that Naruto starts hearing the voices of past Hokages. Maybe they're guiding him in some secret mission or giving him advice on how to deal with new threats in the village. For example, the First Hokage's voice might tell him about ancient sealing techniques to use against a powerful enemy.
It could be his parents. Minato and Kushina might be trying to communicate with him from the afterlife. They could be sharing family secrets or just giving him words of love and encouragement.
Another theme is growth. Naruto's discovery of Sakura's thoughts can lead to his own personal growth. He might become more understanding or empathetic. In some fanfictions, after hearing Sakura, Naruto starts to see her in a new light and tries to be a better friend or potential partner. He might work on his own flaws, like his impulsiveness, in order to be more worthy of her feelings.
It could be that Horton hears a story that has a lot to do with the color blue. Maybe there is a blue character in the story he hears that has some kind of adventure.
The plot could also involve Naruto trying to convince the Raikage to change some of the Cloud Village's more isolationist policies. As Naruto has a vision of unity among all the ninja villages, he sees the Raikage as a key figure to help him achieve this. The Raikage, initially resistant, might gradually come around to Naruto's way of thinking as they spend more time together.
Well, one possibility could be that Naruto somehow gains the ability to perform bankai. He might use it in a battle against a powerful new enemy. For example, he could combine his own ninja skills with the unique powers of bankai to create a whole new combat style that surprises his foes.
Perhaps in this fanfiction, Naruto discovers that he has some unique weapon - like abilities that are different from his normal jutsu. It could lead to a whole new adventure where he has to learn to control this 'weapon' aspect of himself while still being true to his friends and the village.
Well, one possible plot could be that Naruto discovers a new form of chakra that is related to digital energy. He then uses this to hack into systems to prevent a global digital meltdown that some evil hackers are planning. He teams up with his friends, who also have unique digital abilities, and they fight against the bad guys in the virtual world.
One possible plot could be that Naruto is somehow reincarnated as Fenrir. He might retain some of his ninja abilities but also gain the powers of Fenrir like super strength and the ability to change forms. He could then use these combined powers to protect the Hidden Leaf Village from a new and powerful enemy that normal ninja techniques can't defeat.