Compared with everyone else, Kōki actually had one enormous advantage: he already knew the essence of the Six Paths level. That meant he had a direction to follow toward reaching it.
Having read the original story, Kōki suspected that Six Paths power was actually a highly purified, concentrated, and fused form of chakra, one that recorded information about the human body and the characteristics of the soul.
That was why the Sage of Six Paths' sons could reincarnate through chakra, and why his mother could be reborn through chakra within Madara Uchiha.
For that very reason, those who possessed Six Paths power were no longer within the category of humans. They transcended humanity, becoming beings known as immortal sages.
This characteristic was profoundly demonstrated in the Sage of Six Paths, Hagoromo Otsutsuki, and his mother, Otsutsuki Kaguya.
The outward manifestation of Six Paths power was the Truth-Seeking Orbs, which could nullify all ordinary nature-transformation techniques and disintegrate all tissue.
At the same time, those who possessed Six Paths power could easily reverse life and death. In the original story, the Sage of Six Paths, Hagoromo Otsutsuki, had used Six Paths power to save Naruto Uzumaki and Sasuke Uchiha. ()
But obtaining Six Paths power and becoming someone on that level was extremely difficult. In the original story, aside from the Ōtsutsuki mother and her two sons, only Madara Uchiha and Obito Uchiha had reached what could be considered the Six Paths level. Naruto and Sasuke later on could only be considered halfway there.
And their methods of reaching the Six Paths level all involved borrowing external power rather than cultivating it themselves.
As a result, once they were stripped of their source of power, they would fall from the Six Paths realm.
A true Six Paths being would not lose their standing even if they lost their power. And given an opportunity, they could make a comeback.
As far as Kōki knew, there were three or four ways to reach the Six Paths level.
The most direct method was to become the Ten-Tails Jinchuriki. But after becoming its jinchuriki, that power would not belong to him. Unless he could fully integrate it into his body and truly make it his own, he would fall from the Six Paths realm once the Ten-Tails was extracted.
In the original story, Madara Uchiha had been far too careless after reaching the Six Paths level. He had started fighting before properly consolidating his power and elevating his fundamental nature, then lost the fight and ended up right back at square one.
The second method of reaching the Six Paths level was to find a planet, plant a God Tree seed, and eat the fruit once it ripened.
But practically no one could accomplish that. First, they had to obtain a seed from the Ōtsutsuki Clan. Second, they had to find a suitable planet. Third, they had to wait a thousand years.
Only members of the Ōtsutsuki Clan could meet such harsh conditions.
And the second method shared something with the first: one used the God Tree's fruit, while the other used the God Tree itself.
That gave Kōki a conjecture: could an entire God Tree support two Six Paths-level beings?
Otherwise, there was no explaining why both eating the Chakra Fruit and becoming the Ten-Tails Jinchuriki could bring someone to the Six Paths level.
The third method was to study the essence of chakra, fusing ordinary chakra together, evolving it, and elevating it to something approaching the God Tree's fruit. It would then be absorbed by the human body and used to transform it, ultimately bringing the person to the Six Paths level.
This method was also inspired by Otsutsuki Kaguya's absorption of the God Tree's fruit.
Kōki also considered this method more suitable for a ninja like him, who had nothing. It was relatively gentle, too, allowing ample opportunities to test it slowly.
The fourth was to reach the Six Paths level by directly absorbing natural chakra. But this method was entirely Kōki's own speculation. Its chances of success were even lower than the third method's, and it was more dangerous.
Here, he had to address why the Ōtsutsuki Clan had planted the God Tree's fruit in the Shinobi World.
It was because the Shinobi World's environment was filled with natural energy superior to ordinary chakra, but the human body could not absorb that energy directly. They therefore needed a device to convert the violent energy in the environment into gentle energy that the human body could readily absorb.
Only the Chakra Fruit produced through that conversion could allow someone to reach the Six Paths level.
So one thing was certain: natural energy was on the same level as Six Paths power, except that Six Paths power had even more wondrous properties that natural energy lacked.
Then someone might ask why people could not simply absorb the natural energy in their surroundings. That was because humans in the Shinobi World had no method of absorbing energy. Only the Three Great Sage Lands possessed secret techniques for absorbing natural energy, which was why their sages were all so lofty and aloof.
But ordinary people could not reach the Six Paths level by practicing the Three Great Sage Lands' secret techniques. One reason was that those techniques were suited to animal bodies, as could be seen from the animal traits that appeared, to varying degrees, in everyone who practiced senjutsu in the original story.
Another was that, in the original story, successfully learning senjutsu only strengthened a person's ninjutsu and sensory abilities. It did not change their fundamental nature.
Kōki suspected that either the Three Great Sage Lands were withholding their true secret techniques, or they themselves did not know how they had reached the Six Paths level.
Personally, Kōki leaned toward the latter, because the Toad Sage's performance showed that he fell somewhat short of the Sage of Six Paths' realm.
In the original story, Gamamaru knew the cost of the God Tree's presence in the Shinobi World, yet instead of acting himself, he guided Hagoromo Otsutsuki into fighting Otsutsuki Kaguya.
If he possessed that kind of strength himself, why would he need someone else to do it? Wouldn't it have been better to deal with Otsutsuki Kaguya and take care of the God Tree himself?
So, to venture a bold guess, the sages of the Three Great Sage Lands had all slowly absorbed natural energy and reached the Six Paths level over time. They had no special secret technique, only a simple method of absorbing natural energy.
Just thinking about these four methods gave Kōki a bit of a headache.
Not one of them was easy. Even the simplest option, becoming the Ten-Tails Jinchuriki, required defeating Madara Uchiha.
And this method also had that old schemer Black Zetsu watching from the shadows. He would not let anyone ruin his plans.
Besides, even if Kōki really did become the Ten-Tails Jinchuriki, the Sage of Six Paths might personally step in to tell him that some things were beyond his reach.
As for the God Tree's fruit, there was no need to even discuss it. That was completely out of the question.
Natural energy was extremely violent and difficult to test. If one experiment sent him straight to his grave, it would all be over.
The only option with any real possibility was studying the essence of chakra. By fusing chakra of different natures into one, he could create chakra approaching the essence of the God Tree's fruit, then use that special chakra to change his constitution and reach the Six Paths level in one fell swoop.
But this approach of working from the outside inward was not so easy to carry out. It required extensive knowledge and practical experience to support it.
For Kōki as he was now, this method was far beyond him. He did not even have the strength of a Kage yet. Thinking too much about it could easily lead him to aim too high.
And even if he wanted to study chakra's mysteries, he would first have to thoroughly understand the human body. Everyone in the Shinobi World knew that chakra was formed by combining spiritual energy and physical energy.
That meant different constitutions produced different chakra. In the original story, Karin had used the Mind's Eye of the Kagura to sense the differences between each person's chakra.
That meant understanding the essence of the human body and the mysteries of chakra required extensive knowledge of the human body and nature-transformation techniques.
His current store of knowledge fell far short. But Kōki was still young and had plenty of time to experiment slowly.
At the same time, Kōki thought of the quickest way to expand his knowledge: asking Hiruzen Sarutobi for Second Hokage Tobirama Senju's experimental records and training manuals.
In the entire Naruto world, the only two famous ninja with a spirit of exploration were Second Hokage Tobirama Senju and Orochimaru.
For Kōki as he was now, materials related to Tobirama Senju would be a little easier to obtain.
But if he needed inspiration in the future, he could seek out Orochimaru for a discussion. After all, working alone behind closed doors was not advisable.