Maybe Naruto as an old man goes back to fix his own past mistakes. He could try to improve his relationship with Sasuke from the very beginning, preventing all the conflicts that happened between them over the years. He might also use his future skills and knowledge to train his younger self more effectively, making himself an even stronger ninja at a younger age.
One possible plotline could be Naruto using his knowledge from the future to prevent certain tragedies. For example, he might stop the death of a beloved character like Jiraiya.
A possible plotline could be Naruto using his wall - passing ability to infiltrate enemy strongholds undetected. He could gather crucial intelligence or set up surprise attacks. For example, he might slip through the walls of Orochimaru's lair to rescue someone or find out his latest evil plans.
Perhaps the story could focus on Old Man Naruto's relationship with Yamhoto. They could be reminiscing about their adventures together. Yamhoto might be facing some sort of trouble, and Naruto, despite his old age, decides to help him out, showing his loyalty and friendship.
There could be many out there. You could also check Naruto - centered forums. Fans often share their favorite fanfictions there. Sometimes, they even create their own stories and might have written something along the lines of Naruto as an old man traveling to the past. Another option is to look for Naruto fanfiction groups on social media platforms. They might be able to point you in the right direction.
Another plotline could be Naruto influencing the relationships between characters. He could try to bring Sasuke back to the village much earlier in the timeline, preventing a lot of the pain and conflict that occurred between them. Or he could help his parents, Minato and Kushina, in ways that would change the future of the entire Hidden Leaf village.
Maybe in the fanfic, Bella goes to the past and meets Carlisle when he is still struggling with his new - found vampire nature. She could offer him a different perspective on being a vampire, perhaps a more human - centered view. This could lead to a deep connection between them and change the way Carlisle interacts with other vampires in the future. It could also lead to a different relationship between Bella and the Cullen family when she returns to her own time.
One possible plotline could be that Naruto is pushed to the brink during a fight against a powerful and sadistic enemy. Maybe this enemy has been targeting his friends one by one, causing Naruto to gradually lose his sanity as he tries to protect them. He might start using extremely violent and uncharacteristic jutsu, taking risks that he normally wouldn't. Another plotline could be that Naruto is under the influence of a dark genjutsu during a fight. This genjutsu twists his perception of reality, making him believe that his friends are actually enemies, and he goes insane trying to fight them off.
One possible plotline could be about an old man who is a huge Navy fan. He could be facing some legal issues related to his passionate support for the Navy. Maybe he's protesting against a new law that restricts Navy-related activities in his town. And his fierceness comes out as he tries to fight this law.
One possible plotline could be that Naruto is hit by a strange jutsu that causes him to transform into a full - grown man overnight. He then has to deal with the physical and mental changes while still trying to protect the village. His friends might be initially shocked but then support him as he figures out how to reverse the transformation.
One possible theme could be legacy. Naruto, as an old man, might be thinking about the legacy he's leaving behind. Another theme could be change. The ninja world has changed over the years, and Naruto has to adapt to it. Also, friendship could still be a theme. His old friends are still important, and they might support each other in their old age.
Another plotline could be that the crazy old man was wronged in the past and is now seeking revenge in a rather unhinged way. He might be using his old age and the element of surprise to outwit his enemies. He could be creating all sorts of traps and illusions to get back at those who wronged him, and the players get caught in the middle of this revenge plot.