One possible plotline could be Naruto exploring and mastering long - lost Konoha jutsu. He might discover ancient scrolls in the village's archives and start to practice the forgotten techniques. Another could be Naruto teaching the younger generation these special jutsu, passing on the Konoha heritage.
Sasuke could be a rival. Even though they are friends, Sasuke is also a very powerful ninja. He might see Naruto's progress as a Konoha Jutsu Master and feel the need to compete to prove himself. Another could be Neji. Neji is highly skilled and might not be entirely happy with Naruto getting so much attention as a jutsu master.
There could be a jutsu that allows for instant teleportation across long distances. This jutsu might be a new discovery in the fanfiction world, perhaps created by a character who has been experimenting with space - time ninjutsu. They might use it to quickly get in and out of battles or reach far - off places in a blink.
One idea could be a fanfic where a young ninja discovers a unique way to combine lightning jutsu with other elements like water, creating a new and powerful technique. This could lead to him being sought after by different clans for his knowledge.
Maybe Sasuke. Although they are rivals, they also have a deep bond. Sasuke might have come across a unique jutsu during his travels and decide to teach it to Naruto so that they can both become stronger and better protect their village.
There's also the 'Inferno Storm Jutsu' in some fanfictions. It creates a large - scale storm made entirely of fire. The fire in this storm has different intensities, with the center being the hottest and most destructive. It can cover a large area, making it a great offensive move against groups of enemies.
He could practice in secret locations where no one can see him. For example, in an abandoned cave or a hidden part of the forest. That way, no one will know about the new jutsu he's developing.
In fanfiction, if Naruto becomes the master of all jutsu, he would be nearly invincible. He could use any technique to counter his enemies. For example, he could use powerful genjutsu like Tsukuyomi against tough opponents or advanced ninjutsu like Amaterasu. His combat strategy would become extremely diverse and unpredictable.
It could be a new form of Rasengan. Maybe Naruto discovers a way to combine different elements with the Rasengan more effectively, like adding lightning to it for a more powerful and faster attack.
It could also be a new character created specifically for the fanfiction. This character might be an old hermit who has been living in Konoha in secret and is now passing on his knowledge of Fuinjutsu to Naruto or other characters. Maybe he has a special connection to the founding of Konoha and the original development of Fuinjutsu.