Another character that could be involved is Kakashi. He is a skilled ninja and a mentor. He might be teaching the expansion jutsu to the younger ninjas or using it himself in some fan - created scenarios. Also, he could be helping the characters to control and master the jutsu, as he has a lot of knowledge about different jutsus.
Naruto himself is often a main character in such fanfiction as he is the protagonist of the original series and has a knack for learning new jutsus.
Sasuke could also be a main character in 'naruto expansion jutsu fanfiction'. Since he is always striving to gain more power, he might be interested in the expansion jutsu and how it can enhance his combat abilities.
If you want the clearest chapter reference, go to Chapter 164. Early on, Damn, the production team is so shady, they're still doing a live...; later, the chapter leans on Otherwise, I would never have found out..
The best chapter pointer here is Chapter 112, "The Strange "Team Spirit" Appears". It opens with These five people are supposed to be at odds. and closes on [Hahahaha why is the Princess starting to look disappointed?] [Help, even....
A practical answer is Chapter 122, because it turns the idea into a concrete scene. The chapter opens with Damn, are we supposed to find the points card in the mud?!" He had just..., adds pressure through It wasn't intentional!" Bullet screen: "[Qi Tianyu, you really need to be..., and ends on He.
Look to Chapter 122 for the clearest answer. It begins with Damn, are we supposed to find the points card in the mud?!" He had just..., keeps building through To salvage his image, he had deliberately dressed up before leaving the..., and lands on He had just steadied himself when Qi Tianyu.
Chapter 141 is the strongest chapter-level answer because it starts from It had happened in the afternoon when Aunt Ying's son and daughter heard... and follows through on Jiang Li couldn't help but laugh and cry..
The best chapter pointer here is Chapter 126, "The Production Team Actually Played Dirty on This?!". It opens with Wen Jiaojiao initially wanted to cause trouble. and closes on The trolls lurking in the live stream immediately seized the opportunity....
The best chapter pointer here is Chapter 121, "Jiang Li Gains 5 Points for His Meritorious Rescue!". It opens with Seeing that Wen Jiaojiao's condition had improved significantly, Director... and closes on You're supposed to be a man who conquers mountains!] Experienced and....
Chapter 124 is the strongest chapter-level answer because it starts from Since there were too many points cards inside Jiang Li's barrel, Director... and follows through on No need to repeat that number three times.] [Hang on, didn't Jiang Yan....
A practical answer is Chapter 158, because it turns the idea into a concrete scene. The chapter opens with Reluctantly, Jiang Yan had no choice but to forcibly join the group for..., adds pressure through Don't make me laugh, Jiang Li looks better even without makeup than she..., and ends on we're.
A practical answer is Chapter 131, because it turns the idea into a concrete scene. The chapter opens with Just around mealtime, Director Sun came over pushing his small food cart., adds pressure through Just as they tried to emulate Jiang Li's example and make themselves a..., and ends on Hungry.