"When I think it like this, if my Yin Release increases, does that also affect this Sharingan in my left eye socket?" He mused aloud, if his increase of Chakra indeed increases his Yang and Yin Release, then at a higher level, Sharingan should evolve, or its vision should increase as well, right?
This is indeed something that makes him question the possibility of. Just before he parted ways with Zabuza and Haku, he called out Haku, telling him to use his Ice Release in front of him. Zabuza was also surprised, smirked at the thought that Kakashi wanted to copy a Kekkei Genkai now.
However, Kakashi failed, more like his Mangekyō Sharingan failed to completely or fully copy Ice Release Kekkei Genkai. He was able to see how Chakra was moving, how much Water Release and Wind Release in proportion Haku used, however he failed to merge them when personally using the same form as Haku. Understanding the need of creating Ice Release Chakra himself, which requires talent and time like Third Kazekage, Second Tsuchikage, and Pakura had taken to create their respective ones.
"However, if my increase in chakra makes my physical and spiritual energy increased as well, then I didn't need any Kekkei Genkai, but Sage Mode, Senjutsu can be enough later on." He considered, since he thought if Mangekyō can even able to copy Kekkei Genkai, then his strength can increase further itself, but now he feels, that might no longer be needed.
Kekkei Genkai is stronger, Kekkei Tōta is even better, plus Senjutsu is a type of necessity for him. However, if with the increase of Yin and Yang Release Chakra with his overall Chakra, then his left Sharingan eye has the potential to reach the evolution of the Rinnegan.
"If my Chakra in future indeed reaches Ten-Tail's reserves level," Kakashi pondered, "Then my Yang and Yin Release also can be said to be the level of Ashura's Yang and Indra's Yin. Maybe the system has more than just increasing my chakra in numbers, it might have some abilities like this!"
He chuckled at his speculations, if this indeed proven to be true, then later in future he might matches even those Ōtsutsuki's, not in chakra quantity alone but in power as well. Having more Yin Release means his Sharingan's blindness issue will solve, after all Eternal Mangekyō Sharingan at a higher level also about possessing a large amount of Yin Chakra. Plus if this thing doesn't align, then if his Yang Release Chakra matches that of Ashura's Yang, that would also solve the blindness issue of Sharingan.
After all that's how Obito, after transplanting Hashirama's cells from Madara's help, was able to use his Mangekyō Sharingan without caring about losing vision. Hashirama's cells possess Yang Release, which is more potent than what normally ninja have, since he was Ashura's reincarnation, and Kakashi can bypass the quality of Ashura's Yang Chakra, by immense quantity of his overall Chakra Reserves.
"Then all I need to do is wait, and see if my speculation is correct." Kakashi said, lying on his bed, arms behind his head, looking up at the ceiling, with a small smile on his face.
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Three days flew by, and Kakashi had decided to teach Naruto, the Rasengan and Sasuke, the Chidori. Since Team 7 no longer would do D-rank missions, so Sakura didn't even have to come as usual for regular training with Naruto and Sasuke under Kakashi. She can completely focus on medical ninjutsu practices under Medical-nin Kiyoki, which left Naruto and Sasuke alone to train under Kakashi.
"Are you ready to learn A-rank Ninjutsu now?" Kakashi asked, standing in training ground with them, knowing the obvious, glancing with his visible right eye at Naruto on his right-hand side ahead, who was beaming, eyes brightening up with anticipation, and Sasuke on his left-hand side ahead, crossing his arms, but curiosity couldn't be hidden in his eyes.
After all, they had completely mastered their respective B-rank Ninjutsu, plus even done a mission which took them two whole weeks to complete, making them wait impatiently for Kakashi to finally teach them further.
"Of course, we are Kakashi-sensei!!" Naruto said aloud, fists held high, Sasuke also nodded expectedly in agreement.
Kakashi nodded and said, "Alright, now see closely, on the right is the one for you Naruto, it's called Rasengan, and the left one is for you Sasuke, it's called Chidori, my own created."
He took both his hands out forward, in his right hand, he focused his chakra, a blue spherical ball rotated, of around 20 cm in size, and on his left hand, electricity sparkled, sounding like thousands of birds chirping.
"Um, Kakashi-sensei, is this ball really an A-rank Ninjutsu, the one you called one of the signature moves of the Fourth Hokage?" Naruto said, with little unimpressed tone, "Plus that lightning one feels more powerful."
Kakashi smiled under his mask, since upon seeing and hearing the sound of Chidori, it indeed makes it seems more intimidating and powerful than Rasengan.
"Actually, Rasengan is the one, from which I was able to create Chidori, plus it's very powerful," Kakashi explained, slammed his right hand on the ground, creating a huge crater beneath them. "Plus this is also not a complete but an incomplete Jutsu, it can still evolve further, also same is with Chidori."
Naruto and Sasuke looked beneath them, in surprise, that silent-looking blue ball, created a huge hole in the ground. Naruto now no longer have the doubt that Rasengan was going to be any weaker than Chidori, while Sasuke also didn't doubt that Chidori was weaker than Rasengan, despite the impact, otherwise he believed Kakashi wouldn't teach it to him.