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.
Hashirama's revival might also bring back old wisdom and knowledge. He could teach the new generation of shinobi things that have been lost over time. However, there could also be political issues. Some might not be happy with a long - dead Hokage coming back and potentially interfering in current affairs. And his revival could also have an impact on Naruto's own growth. With Hashirama around, Naruto might have to re - evaluate his role as a leader and a shinobi.
In some fanfictions, Naruto might use a powerful jutsu. Maybe he discovers an ancient sealing technique that can bring back the dead for a short time. He could also have found some rare chakra-infused artifact that has the power to revive the First Hokage.
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.
The cells can also introduce internal conflicts. A character might struggle with the foreign power within them. They could be afraid of losing control, like Orochimaru was with his various experiments. This internal struggle is a great way to develop the character's emotional side and show their insecurities.
To develop Hashirama's ghost, we could give him a sense of purpose. He could be a protector of the village in the afterlife. So, when he senses a great danger approaching the village, he decides to reach out to Naruto. He could have a deep connection to the natural world even as a ghost, and his powers could be related to manipulating nature spirits. He could teach Naruto how to communicate with these spirits and gain their assistance in times of need.
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.
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.
It could be Naruto himself who discovers and uses the Katon Hijingakure no Jutsu in the fanfiction. Since he's always exploring new jutsus and pushing his limits.