After settling down near the God Tree, Hachiman started meeting Kaguya everyday; as time went on, they got more and more familiar with each other, and a year later, Hachiman had already started the construction of a city that worshipped the Rabbit Goddess, Guardian of the Divine Tree of Life.
Surprisingly, Kaguya was quite supportive of his idea of building a village, it seemed that she really was quite bored or just stressed about her clan coming after her, so she welcomed any distractions.
Then logically, Hachiman used his identity as the Goddess's emissary to be the village chief and control everything within it–of course, this whole idea wasn't just for fun, he planned to gather many "loyal" followers and gift them inherited curse marks akin to the Hyuga's Caged Bird seal so they could serve him wholeheartedly, it was fundamental to acquire some subordinates and test subjects for the the sake of his future research.
Hachiman and Kaguya were actually quite open with each other as time went on, she pretty much told him her entire life story and he told her about all his plans, some of which Kaguya found quite ingenious and others, well, she has learned to ignore some of the nonsense he says only around her; he has even said he is also an alien that came here carrying a vision of the future and that they must get married to solve their crisis, it was clearly a lie, but for some she couldn't see a trace of trickery in his eyes when he said those words.
While he initially made many preparations for the future, Hachiman realized they were just that, for the future–so he quickly reached a point where he couldn't do much else except for ever so slowly increasing his strength and starting preparations for future experiments.
Although Hachiman and Kaguya were now getting close, he would never really try to convince her to directly give him her own power, especially when he knew he just couldn't ever convince her; she believed humans should not have the ability to use chakra, and that they were better off giving her all the chakra so she could resist her family's pursuit herself, and as of now, Hachiman really couldn't persuade her to change her mind, he could only ask her to not randomly choose to one day turn the entire world into her army without warning.
As time continued to flow, old leaves fell, and new leaves sprouted anew, in short, after five more years, Hachiman was now 24 and Kaguya was pregnant with their twins.
Neither of them were sure of when it happened, but somehow they went from one side occasionally barking at the other with exaggerated love declarations and the other side remaining mostly skeptical and indifferent, to then developing a real relationship with each other–now, they finally confirmed a human and an Otsutsuki could successfully generate offspring: and for some reason Hachiman secretly used this as a justification to not see Kaguya as an alien, after having kids, clearly she is the same species as him, not a dirty immigrant that came to steal their chakra.
But with kids on the way, Hachiman was forced to remember Kaguya's original pair of filial children that sealed her away on the Moon, and when he thought of the reason they sealed her, he realized his existence hadn't changed anything, his strength was so meager compared to Kaguya's that he gave her no reassurance at all, she was still hellbent on executing the Infinite Tsukuyomi plan, having only delayed it because she was content with staying like this for a few more years.
But now, these children are definitely going to give her a sense of urgency and a desire to protect them, she won't stop or scale down the Infinite Tsukuyomi even if he suggested it to her, because every time she asked if he had a better option, he could only say they would be able to find a better way in the future, which left both parties unsatisfied.
With this in mind, Hachiman decided to prepare for the worst and most likely scenario, his wife and children ignoring his persuasion and the original plot repeating itself–and now that he thought more clearly about it, Kaguya seemed almost obsessed with the Infinite Tsukuyomi plan, as if that were her the natural course of things, he started to suspect the chakra fruit she ate made her subconsciously seek ways to acquire more chakra, maybe this chakra addiction is one of the reasons the Otsutsuki keep harvesting planets so relentlessly.
The more he thought about it, the more he thought it was possible, as chakra was clearly shown to be able to store emotions or even have it's own will, so wouldn't it make perfect sense for an entire planet's chakra to affect the consumer's desires?
Truthfully speaking, this consideration made Hachiman feel a little helpless, he simply didn't have enough time to grow his strength, and worse yet, this God Tree was clearly abnormal according to what he remembered reading online, so besides Isshiki and the future Otsutsuki envoys, he suspects Jashin and Otsutsuki Shibai to be the two biggest wildcards that could be involved with this world.
The existence of Jashin was never properly explained, but Hachi suspected he must be some sort of rogue Otsutsuki or a similar being using blood rituals to gain power instead of chakra fruits, and if that's the case, that's another dangerous individual that's watching this clearly abnormal world.
As for Otsutsuki Shibai, the possibilities are endless, but he knew with certainty that his discarded body either ended up in the hands of Isshiki before coming here, or that he found it here on Earth under unknown circumstances–either way, the existence of Shibai related things on Earth complicates things even more, proving this planet is definitely abnormal.
No matter what conjectures Hachiman came up with, they wouldn't be too relevant for at least at thousand years, he has already completely eliminated Isshiki in his current pathetic state with the help of Kaguya, the "fight" was extremely underwhelming, it was an immediate execution with an all-killing @
ash bone sneak attack.
With that in mind, Hachiman gave his all to taking care of Kaguya during her pregnancy, in truth, a being like her could give birth at any moment as long as she felt like it, and pregnancy wasn't a big deal, but she still shamelessly played along with it for two years as she had started to like the princess treatment.
At age 26, Hachiman finally became a father of the twins Hagoromo and Homura Otsutsuki–yes, he decided he might as well let Kaguya choose their name and surname; he couldn't help it, when he saw the way she looked at their children, Hachiman could almost physically see the love she felt for them, which made him feel even worse about her fate.
Don't misunderstand, Hachiman wasn't just letting things happen out of cowardice, he repeatedly tries to convince Kaguya to change her ways and warns her of the risks if their children end up siding with humanity if she tries to execute her plan after they grew up here, but she is too confident in her children's love for her and her own strength to consider them ever going against her, and since Hachiman truly doesn't have any short-term plans to deal with the Otsutsuki, there's no changing this woman's mind, he can only watch history repeat itself.
And after a relatively harmonious two decades of raising the two little boys in their village, the twins decided to go out to see the world for themselves, what they were most eager about was escaping the loving fists of their father.
Keep in mind that with Hachiman's teachings, not only was Kaguya a much better fighter, but even Hagoromo and Homura had much better combat skills than their original counterparts; the issue was that just as Hachiman suspected, his unfilial children had the same stubbornness their mother did, so he simply gave up on changing their fate and decided to just vent a little and prepare for it, at this point he had already been conditioned to be more submissive towards Kaguya, she had done many interesting things with him using her hair bending and flight capabilities, he no longer had any thoughts of ever being the dominant one in the relationship, only he was mouth was as bold as ever, but it was fine, he enjoyed it either way, as long as no one ever knows of how he gets essentially manhandled, it's actually quite great.
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Hachiman knew he was getting old and his strength had stagnated, his skills like fuinjutsu, bukijutsu, and kenjutsu had reached the limit of his capabilities, he could now be considered a grandmaster in all of these and could kill anyone in the history of regular shinobi, except for the prime versions of his grandkids' last two reincarnations and maybe that snake-like cockroach that loves licking children.
His chakra control was also god-like at this point, he even managed to recreate all the B-rank or lower jutsu he could think of, he also developed many standard element fusion kekkei genkai such as Ice Release, Lava Release, Scorch Release or Magnet Release, even though he only developed most of them in the last couples of years, but special ones like Wood Release that require immense vitality and Yang chakra or kekkei genkai that involved the body were out of reach for his current self, even the Dark Steel Release still elude him, he only knows Steel Release must use Earth nature chakra, but with no information he still hasn't succeeded with either; he is also on his way to developing Dust release and another kekkei tota he named Star Release, all he needs is more chakra to properly use the techniques, as for A-rank and S-rank jutsu, it once again boiled down to chakra, he could recreate Rasengan and Chidori for example, but other jutsus weren't nearly as intuitive, so he couldn't waste his time with trial and error.
His progress on utilizing Natural Energy was still the same, zero. Even though he assigned a team of subordinates to help him research ways to safely use Natural Energy, it was ultimately flawed as the experimental subjects weren't even compatible with Sage Arts or proficient in chakra use. Not even the few White Zetsu he painstakingly managed to get from Kaguya helped much, they only serves to show him a path to obtain a unique body at the cost of becoming a plant human, but that's not what he wanted, although he was definitely going to copy the Mayfly and Universal Assimilation techniques someday.
As for fuinjutsu, what can he say, without being an Uzumaki, all he could do was use seals about as well as the likes of Hiruzen, Danzo, Jiraiya, Minato, Orochimaru, etc.
Besides his Stairway to Godhood technique,that still had permanent side effect if he wanted to defeat someone like Hashirama with it, Hachiman had spent the majority of his time trying to develop a space-time jutsu of his own, but even with Kaguya's guidance he could only replicate the Flying Thunder God technique, which, while not useless, wasn't necessary for him with his Swift Release and it wasn't anything special at the Six Paths level or above. He was still about as good as Minato ever was with it though, the difference was that he used Magnet Release to spread iron dust marked with his Raijin Seal, which made up a deadly combo.
As for him main problem of not having enough chakra, he already had a way to resolve it, the issue was that this was a one-time process, and ignoring all the preparations he had to do, his other problem was the need to stay stuck to the God Tree for as many years as possible to achieve the desired effect.
This meant that after all these years of working tirelessly as the Shinobi incarnation of Leonardo Da Vinci, seriously, he's head priest, mayor, chief executive/financial/technological officer, full-time shinobi, training coach, loving husband, and hands-on father, and before you ask, no, everybody knows Hagoromo and Homura flinching whenever he raises his hand has nothing to do with him.
But even if Da Vinci must applaud his efforts, Hachi's still going to have to watch everything he's worked so hard for come to an end and isolate himself from the world for a thousand years just like his wife and two sons.
When Hachi finally solidified his resolve to let things follow their course, so he went to see Kaguya and tell her about his long-term plan–the funny thing is she had actually offered to split her own power with him because she really couldn't bear to see him slowly age as she remained youthful; but he always said it was fine and that he had a solution in mind, so now was the time to reveal what he had been preparing for.