It could be a jutsu that manipulates life and death. Naruto has always been about protecting life, but this jutsu might give him the power to bring the dead back to life. However, this would go against the natural order of things in the Naruto universe.
Well, he could become extremely powerful. Maybe he'd use the forbidden jutsu to defeat powerful enemies like Madara or Kaguya in a really unexpected way. For example, if it was a time - travel related jutsu, he could go back in time and stop certain events from happening that led to the rise of these villains in the first place.
Some fanfictions might explore the idea of combining Forbidden Jutsus. Picture a scenario where a rogue ninja combines a Forbidden Jutsu of destruction with a Forbidden Jutsu of mind control. This could create a super - powerful and extremely dangerous new ability, and the story could follow the efforts of the main characters to stop this new threat.
Naruto is often involved. Since he is the main character of the original series, it's natural that in fanfiction about Forbidden Jutsu, he would be curious or accidentally get involved with these powerful techniques. Sasuke is another common character. His thirst for power could lead him to seek out Forbidden Jutsus.
He could create a jutsu like 'Blood Clone Jutsu'. Using Alucard's ability to manipulate blood, Naruto could create clones made of blood that have the same strength and abilities as him. These clones would be more durable than his regular shadow clones and could even explode on contact, causing damage with the blood splatter.
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.
He could make time - related jutsus. For instance, a jutsu that can slow down time in a small area, which would be really useful in battles. This is something that isn't in the original Naruto but is possible in fanfiction.
Well, one interesting concept could be the unique ways the forbidden jutsu are reinvented. For example, some fanfics might explore how a forbidden jutsu could be modified to have different effects or be used in unexpected situations. Maybe a forbidden jutsu that was originally for destruction is tweaked to be a form of protection in a fanfic.
He might learn Orochimaru's signature snake - related jutsu, like the Hidden Shadow Snake Hands. This jutsu allows the user to summon snakes from their sleeves or other hiding places to attack opponents.
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.
She could teach him a genjutsu - based jutsu. For instance, a genjutsu that makes the opponent see multiple copies of Naruto, confusing them about which one is real. This would be useful in the Chunin Exams as it can give Naruto an edge in combat.