He might go into hiding for a while and train intensively. He could come out with new and improved skills that shock everyone. For example, he could master a new form of Rasengan that is even more powerful. Then he might start to build an underground network of ninjas who believe in his cause and are against the unjust banishment.
There could be a mysterious mentor figure who guides Naruto in his secret life. This person might have unique abilities and a deep understanding of the ninja world's secrets. Another potential new character could be a spy from another village who discovers Naruto's secret life and tries to use it to their advantage.
Perhaps an old, powerful shinobi who was thought to be dead but has now returned in Naruto's new life. This shinobi could be seeking to reshape the ninja world according to his own dark vision, and Naruto would have to stop him. The antagonist might have some connection to Naruto's past life that is gradually revealed as the story progresses.
Well, their new life in fanfiction could be really interesting. Naruto, being the Hokage, could finally have some peaceful time with Sakura. They could open a ninja training academy together. Sakura would be in charge of the medical training part, while Naruto would teach combat skills. And they could also go on missions from time to time just for the fun of it, like exploring some ancient ninja ruins.
One idea could be Naruto being reborn into a different clan, like the Uchiha, and having to deal with the clan's secrets and rivalries from a new perspective. Another could be him starting his ninja training much earlier in this new life, leading to different power developments. Maybe he has a different set of friends or allies in this new life as well.
I've read a few 'New Life Naruto Soul Eater Fanfic'. In one, Naruto's determination from his previous life in Konoha made him strive to become a top - tier meister in Soul Eater. He had to adapt to the different power systems, like learning about soul wavelengths instead of chakra control. His journey was filled with challenges as he faced off against powerful Kishin - egg - like villains, and it was really cool to see how he combined his ninja skills with Soul Eater techniques.
One idea could be a fanfic that focuses on the untold adventures of Naruto's parents, Minato and Kushina. It could explore their relationship, their time as ninja in Konoha before Naruto was born, and how their actions and decisions shaped the future of the village.
Naruto could be found by Orochimaru. This would be a very dark turn. Orochimaru could use Naruto's potential for his own evil plans. Naruto would be trained in Orochimaru's unethical ways, and his view of the ninja world would be completely distorted. He might become a villain or at least an anti - hero, very different from the Naruto we know in the original story.
One popular plot twist could be a character from the original Naruto series having a completely different backstory. For example, Sasuke might be revealed to have a secret connection to an ancient clan other than the Uchiha. Another is when a new character is introduced who has the power to change the entire shinobi world order. And sometimes, Naruto himself might gain a new and unexpected power that is not related to the Nine - Tails, like the ability to control time on a small scale.