Jiraiya could be one. Although he is known more for his fire and earth - based jutsu, he has vast knowledge and could guide Naruto in using water affinity in unique ways. Another could be Kisame. He is a master of water jutsu. His skills with water manipulation are top - notch, and he could teach Naruto some really powerful and advanced water techniques.
One cool jutsu could be the Water Spiral. Naruto would spin water around his body at a high speed, creating a protective and offensive spiral. Another could be the Water Mirror Clone. He creates clones made of water that can reflect attacks back at the enemy. And there's the possibility of the Water Tsunami, where he summons a huge wave of water to overwhelm his foes.
One possibility could be that Naruto discovers a hidden ability within himself to use Wood - Style Treant Jutsu. Maybe he unlocks it during a life - threatening battle where he needs to protect his friends. He could combine his own chakra with some ancient energy source in the forest to create these powerful treant - like creatures that fight on his side.
The Water Dragon Jutsu could be modified to carry other elements within it. For example, the user could infuse lightning chakra into the water dragon. So when the dragon attacks, it not only hits with the force of water but also delivers electric shocks. This would be a great new battle strategy as it combines two different elements for a more powerful and unexpected attack.
Maybe in the fanfiction, the Water Dragon Jutsu has been corrupted by an evil force. Naruto and his friends have to find a way to purify it. They might need to travel to an ancient water temple where they face various trials and battles with dark water creatures that were created due to the jutsu's corruption.
No, Boruto is alive in the manga. The plot hasn't led to his death. However, the manga is ongoing, and anything could happen in the later developments.
A 'Solar - Flare Rasengan'. Naruto could combine his wind - style chakra with some kind of solar - based energy he discovers. This jutsu would be like a super - charged Rasengan that emits a blinding light as well as powerful wind - based damage.
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.
The fanfic could start with Naruto having a strange dream about using Wood - Style Treant Jutsu. He wakes up curious and starts to experiment. He finds that he can transfer his own emotions into the treants, making them more aggressive or defensive depending on how he feels. As he gets better at it, he uses the jutsu in a major battle against a group of rogue ninjas who are terrorizing a village. His treants become a symbol of hope for the villagers as they fight off the invaders.