One challenge could be controlling the natural energy. Since senjutsu involves harnessing natural energy, Naruto might find it difficult to balance it with his own chakra. It's like trying to mix two very different substances without causing an explosion.
Well, in some fanfictions, Naruto might start his senjutsu learning journey by stumbling upon a hidden sage or a mysterious place. Maybe he's on a mission and gets separated from his team, and that's when he discovers a clue about senjutsu. Then he follows that lead, driven by his curiosity and his desire to become stronger.
Well, it significantly boosts his power. Senjutsu gives him new chakra control and more powerful jutsu. With Kurama's help in learning it, Naruto can combine his already vast chakra reserve with senjutsu chakra, making his attacks much stronger and more unpredictable. For example, his Rasengan could be enhanced with senjutsu, making it more destructive and harder to defend against.
In such a fanfiction, we might see Naruto using his senjutsu to gain an edge in the battles that often occur in the High School DxD universe. His sage mode could give him an advantage against the various powerful beings in that world. The fanfiction could also delve into the relationships between Naruto and the different characters in DxD. Would he become friends with Issei? How would Rias react to Naruto's presence? There are countless possibilities for an exciting and unique story.
One challenge is controlling the amount of nature energy. It's not easy to balance it with his own chakra. Also, Kurama's chakra can be a bit wild, and combining it with senjutsu chakra requires a lot of focus. Another thing is that Naruto has to learn new techniques quickly, which can be mentally taxing.
Naruto himself would be very interesting. With his senjutsu skills, he would be a force to be reckoned with in the Chunin Exams. His growth and how he uses his newfound powers in different situations would be exciting to follow.
A traveling nen master could be Naruto's mentor. This master could be on a journey to spread the knowledge of nen. He crosses paths with Naruto and decides to teach him. The master might have a strict teaching style, pushing Naruto to his limits, but also guiding him carefully through the complex world of nen.
There's a possibility that it's a spirit or a deity in the world of bending. This entity is impressed by Naruto's tenacity and chooses to guide him on his journey of learning bending.
It could be an old hermit who has been living in seclusion for years. This hermit has mastered the Goken and sees potential in Naruto. So, he decides to teach Naruto this powerful technique.
A common power is enhanced physical abilities. They can be much stronger, faster, and more durable. For example, they might be able to jump extremely high or run at incredible speeds.