He might learn a time - traveling jutsu. This would be really cool as it could allow him to go back and change certain events in the Naruto world, or gain knowledge from the past to use in the present battles.
Well, one key element is the mentor. In many of these fanfictions, Kakashi or another experienced ninja acts as the mentor. They guide Naruto through the process of understanding and controlling the lightning - based chakra. Another element is Naruto's motivation. Usually, it's either to become stronger to protect his friends or to prove himself to his village. For example, in some stories, Naruto wants to show that he can be as powerful as the other top - tier ninjas in the village.
In fanfiction, Naruto learning the Vacuum Jutsu could be really cool. It might give him a new and powerful offensive technique. Maybe he'd learn it from an unexpected source, like an old hermit in a forgotten land. This could potentially change the power dynamics in battles he's in.
He could potentially learn the Flying Thunder God Technique. It's a very powerful jutsu that Minato was famous for. Naruto's already fast with his Sage Mode and Nine - Tails Chakra, and this would make him even more formidable in battle.
In some fanfictions, it could be an enemy-turned - ally. This character, who originally fought against Naruto, realizes his strength and potential and decides to teach him black lightning as a way to make amends or to join forces with him for a greater cause.
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.
In fanfiction, the Lightning Dragon Jutsu can be extremely powerful. It can be depicted as a massive dragon made of pure lightning that can obliterate anything in its path. Another powerful one could be the Lightning God's Wrath, a jutsu that summons a bolt of lightning from the heavens with the power to level mountains.
Well, in many fanfictions, Naruto might learn black lightning through intense training. Maybe he gets some special guidance from a powerful shinobi who has mastered it, like Kakashi or some other unknown but strong character. He could also discover some ancient scrolls that contain the secrets of black lightning.
He could develop a 'Lightning Clone Jutsu'. Instead of regular clones, these would be made of lightning. They could move at high speeds and deliver electric shocks on contact. Another jutsu could be 'Lightning Shield', where he surrounds himself with a layer of lightning for both offense and defense.