Another plotline could be Naruto's relationships in adulthood. For example, his friendship with Sasuke could face new challenges as they have different responsibilities. Also, his relationship with his family, like how he raises his children and imparts the values he learned throughout his life.
One interesting plotline could be Naruto dealing with the complex political landscape of the Hidden Leaf as an adult. He might have to make tough decisions regarding alliances and the future of the village.
One interesting plotline could be the mystery behind the mask. Maybe it has some special power that only Naruto can unlock or it could be related to an ancient prophecy. Another could be a new villain who uses the mask to create chaos in the ninja world and Naruto has to stop him.
Perhaps Naruto discovers a new medical jutsu that combines his chakra control with traditional medical knowledge. This could revolutionize the medical field in the Naruto universe. He might use it to heal previously incurable diseases or injuries, and in the process, gain the respect of the medical community. This could also lead to some interesting interactions with other medical ninja, like Shizune or Tsunade.
Well, in some fanfics, there's a plotline where Hinata loses her memory and Naruto has to make her fall in love with him all over again. It's a classic but still very engaging. There are also stories where they are arranged to be married for political reasons at first, but then they actually fall in love for real during the process. And a really unique one is when Hinata becomes a super powerful ninja overnight and Naruto has to deal with his feelings of being a bit overshadowed while also being proud of her.
One interesting plotline could be Naruto joining the Akatsuki and trying to change it from the inside. He might use his ideals of friendship and peace to influence the other members who have mostly known a life of darkness and violence.
One interesting plotline could be Shizune's involvement in secret medical research within the Hidden Leaf. Maybe she discovers a new form of medical ninjutsu that could change the way they treat serious injuries. Another could be her relationship with Naruto from a medical - training perspective. She helps him understand the importance of medical skills for a ninja, and they bond over that.
One plotline could be Scathach training Naruto in some ancient and powerful techniques that are not known in the ninja world. This would give Naruto an edge over his enemies and also lead to interesting interactions between them as Naruto tries to master these new skills.
A possible plotline is Naruto using his unorthodox methods to solve ANBU - related problems. His creativity and ability to think outside the box could lead to new strategies for the ANBU. He might come up with a new way to gather intelligence or take on a powerful enemy that the traditional ANBU methods couldn't handle.
One interesting plotline could be Naruto joining the Kumo village and having to adapt to their different ninja techniques and traditions. He might face challenges from the Kumo ninjas who are initially suspicious of him.
One possible plotline could be Naruto getting caught in a blizzard during a mission and suffering from hypothermia. His friends then have to race against time to find and warm him up. Maybe Sasuke uses his Sharingan to detect Naruto's chakra in the snowstorm.
One possible plotline could be Naruto being sent on a special mission to Kiri. He might encounter unique Kiri-nin and their special jutsus, and have to form unexpected alliances to complete his mission.