Perhaps an unknown hermit living in some far - off place. In many fanfics, there are mysterious characters that pop up with unique knowledge. This hermit could have mastered the Reaper Death Seal long ago and sees Naruto's potential, so decides to teach him.
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.
Power and sacrifice are common themes. The death seal is a very powerful jutsu that often requires sacrifice, so many fanfictions explore this aspect. For example, a character might have to sacrifice their own life or something precious to use the seal effectively.
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.
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.
A rogue shinobi who has found a way to corrupt the Reaper Death Seal and use it for his own evil purposes. He could be stealing the souls of innocent people instead of using it as a last - resort jutsu like it was intended.
In this fanfic, Naruto might face some really tough challenges while learning the Reaper Death Seal. He could be in a situation where he has to sacrifice a lot to master it, like maybe losing some of his relationships temporarily. Also, he might have some sort of spiritual journey to fully understand the power of the seal.
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.
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.
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.