With the character of Toyosatomimi no Miko being inspired by multiple semi-legendary tales about Prince Shoutoku, and considering some other things going on in Touhou canon, I'm surprised that nobody has yet come up with a story about Miko having some really intense inner conflicts between different facets of herself and/or between Miko and all those versions and interpretations of the historical figure that was Shoutoku.
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Miko in the Touhou universe is meant to be an alternate version of Shoutoku that secretly practised Taoism, rejecting both Buddhism and Shintou in the 6th century AD, a time when Japanese society was divided along the lines of those two religions as part of a civil war and Shoutoku himself became famous as a military leader of said civil war who brought Buddhism to Japan.
I found that Miko fully embracing Taoism and practising Taoist arts to attain immortality could be related to a desire to be her own person and grow, not only above herself, but also above all the myths surrounding Shoutoku and all the interpretations others made about his life story and his feats. In turn, this resonates with Miko's characterisation as a canon transgender female character in Touhou, including her decision to use her reincarnation as a shikaisen to undergo a complete magic-powered gender reassignment therapy.
Moreover, I have a headcanon that one of the reasons why Miko is rejecting Buddhism so ardently is because it's a reminder of what she used to be perceived as in her previous life, the identity imposed on her that she's trying to escape from. Now, it doesn't need to be this way, and besides, neither Byakuren Hijiri nor any of her followers at Myouren Temple has any idea that this is going on in Miko's mind. This makes her situation all the more tragic.
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However, Shoutoku's ties to Buddhism are still a fundamental –even inescapable– part of Miko's very self, due to the whole thing about belief being what fuels the existence of Gensoukyou and its inhabitants (or at least, those inhabitants of Gensoukyou who aren't people from the Human Village).
She's not just vaguely based on the historical figure of Shoutoku and the various tales surrounding him, but she's heavily inspired by one specific version: Guze Kannon, the interpretation of Shoutoku as a messianic reincarnation of the bodhisattva of compassion Avalokiteśvara (Guān Yīn in Chinese, Kannon in Japanese), according to the devotional cult that developed around the Crown Prince for a while since a century after his death (as pointed out by @sorcerorsutrascroll here: https://sorcerorsutrascroll.tumblr.com/post/618837241216843776).
Now, it should be possible for Miko to conciliate Taoism, whose teachings she openly follows at present and secretly followed in her past life, with Buddhism, whose principles she only publicly adhered to as Shoutoku in the past but still partly define her very existence in her present life as part of Gensoukyou; after all, Shingon and other Japanese Esoteric schools of Buddhism were greatly influenced by Taoism, as pointed out by @just9art in this post [https://just9art.tumblr.com/post/793045036761858048] (by the way, Esoteric Buddhism in general and Shingon in particular also seem to be the branch of Buddhism with the most influence in general Touhou world-building, in comparison to more "conventional" Mahāyāna schools such as Zen), and Shoutoku himself used the Seven-Star Sword, a relic associated with Taoism (and the ceremonial sword where Miko placed her soul for her reincarnation process). Doing this conciliation between both religions might even help Miko personally in both her pursuit of immortality and the reaffirmation of her identity; after all, as pointed out in the post from earlier by @sorcerorsutrascroll, the Sūtra of Instructions of the Taintless Flame (on which Shoutoku is even said to have written commentary) teaches there isn't any inherent or fundamental difference or separation between man and woman (understanding this can be used as a path towards rejecting the gender binary as a supposedly undeniable and inescapable biological truth, understanding gender as a spectrum, and all this stuff that queer studies scholars independently figured out again since the 1970s).
However, if Miko's motivation to choose Taoism includes seeking to –as described earlier– be her own person, grow beyond her old Shoutoku persona and surpass all the ideas and interpretations people assigned to and projected on her as Shoutoku, then she'd never do any of these things to follow Taoism and Buddhism at the same time, or even entertain the idea of admitting Buddhism is right about something. As I mentioned earlier, this makes Miko's attitude even more tragic because (as seen in this post [https://mashounen2003.tumblr.com/post/697974519511515136] by @sweetescapeartist –and my Spanish translation of it– about Krillin overcoming his PTSD by attaining enlightenment in the Dragon Ball Super anime) Buddhism can also be a path to grow beyond your own limitations and healing from past traumas, something that would align a lot with Miko's personal goals.
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On top of all this, aside from the Taoist and Buddhist facets of Miko, there's also a Shintou facet at play. Well... The issue here isn't really Shintou as a whole, but a very specific (and relatively recent) part of it: the Heavenly Kami, and more precisely, the Japanese Imperial Cult, which focused a lot on the Heavenly Kami as a source of authority. This is because, in the Touhou universe, Miko still has a connection to the Lunarians.
Shoutoku played a vital role in the arrival of Buddhism in Japan, and Miko currently follows Taoism; however, Shoutoku was still a key member of the Japanese imperial family, and Miko insists on using the title of Crown Prince. The imperial family always claimed to be direct descendants of the Heavenly Kami; even with all the changes in their relationship with religions other than Shintou (for example: in the 6th and 7th century, they were accepting and even promoting Buddhism; more than a millenium later, in the late 19th century, Emperor Meiji ordered a strict separation between Shintou and Buddhism, invented an alleged reconstruction of the "original" Shintou that was supposedly "free of all foreign influence", and declared this "State Shintou" as the Japanese nation's new official religion), these claims of divine ancestry remained a constant throughout the imperial family's entire history, and it's logical to assume this belief also applied to Shoutoku back when he lived.
In the Touhou universe, the Lunarians see themselves as the Heavenly Kami, and the Lunar Capital's feelings of superiority and obsession with purity are mirroring –in a very direct and unsubtle way– the mix of bigotry, xenophobia, extreme nationalism and religious fanaticism that defined the Japanese Empire in the late 19th and early 20th centuries (and has never truly gone away after that) and was the ideological basis for policies such as the creation of State Shintou and the Buddhism-Shintou separation ordered by Emperor Meiji.
Let's go back to Miko. After she had grown to sincerely respect Byakuren and had been willing to team up with her in Touhou 15.5, Miko stopped appearing in new games and only kept showing up in official print works. In said print works, she started being portrayed around 2016 as someone who not only strangely believes in a firm separation between Japanese and other East Asian religions that used to be very intermingled and syncretised for over a dozen centuries, but also has got what I could only describe as a "human-supremacist streak" (Aya might have exaggerated this in the article about her in Alternative Facts in Eastern Utopia, but quite a few things Miko said afterwards kind of proved in retrospect that Aya was correct by accident about this one thing).
In a similar way to how Reimu Hakurei behaved a bit more like the Lunarians for a long while in the 2010s in a desperate attempt to fulfill the duties of a traditional shrine maiden [https://mashounen2003.tumblr.com/post/809576855521853440], it's possible that the Meiji-era fanaticism embodied by the Lunar Capital started to affect Miko's personality as well, via the belief that the Lunarians are Shoutoku's divine ancestors and Miko –both as a reincarnation of Shoutoku and as the Crown Prince she still claims to be– should follow their example.
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It'd be fantastic to see a story where Miko's own character arc comes to a close in the context of Gensoukyou's conflict with the Lunarians. This could have the many facets of her personality fighting against each other, and I can think of two different ways this could go: one of them is those facets taking their own separate forms in the physical plane, with Miko having to put all of them under control and reunite with them; the other possibility is that the clash between Miko's self and all these components of her identity stays in her mind and she starts physically and mentally deteriorating as a side-effect. Miko would eventually end this inner conflict and overcome this crisis with advice from all of her friends, which provides her with perspective and helps her fully discover herself and figure out what she really is:
Soga no Tojiko and Mononobe no Futo. Both of them embarked on the same journey to attain immortality. Futo's point of view in particular would be interesting to explore: aside from her having originally been a follower of Shintou, there's the fact that she's so loyal to Miko but also used to be the first one to propose attacking the Myouren Temple, and this kind of animosity towards Buddhism is what Miko would need to abandon.
Byakuren, along with the Myouren Temple crew. Given what I've described about the possible connection between Miko's rejection of Buddhism and her issues with her past life as someone different, I imagine this conversation would be the most emotional since it'd require unpacking all that baggage she's been bottling; it'd also be rather ironic if Miko, someone worshipped at one point as a living deity of mercy, was the one on the receiving end of said mercy.
Reimu and/or someone from the Moriya Shrine, most likely Kanako. This would be important because it'd show what Shintou has always been at a fundamental level, what it literally was before becoming an organised/institutional national religion for Japan: a form of Animism, a belief system that focuses on respecting, appreciating and connecting with the essence of nature, the universe we live in and all its interconnected elements (the definition of the Kami –in singular– in its most basic interpretation, and also the basis for the Earthly Kami –in plural– such as Suwa Myoujin, whose lore is the inspiration for Kanako and Suwako), regardless of any big complicated family tree of heavenly beings with their own wills or personal goals. The conversation with Kanako would be relevant, not only due to the question of the interaction between religions (Kanako's father/grandfather Oukuninushi was syncretised with the Buddhist deity Daikokuten; a bit more about that here [https://mashounen2003.tumblr.com/post/809375294976851968] and here [https://mashounen2003.tumblr.com/post/808569429877227520]), but also because she initially became a kami after being born as a human (something not too different from Miko and her fellow Taoists becoming shikaisen) and now seeks to become a kami of innovation, which could also inspire Miko; Suwako, as a native god, would also help pointing out what I've just described about the Animist roots of Shintou. As for Reimu, I've already talked about her in that post I linked earlier about her portrayal in the 2010s: after the events of Touhou 19 & 20 and Whispered Oracle of Hakurei Shrine, she'd be in a good position to explain what Gensoukyou is about (acceptance, evolution, infinite possibilities) and that the Lunarians are dangerous due to how easy it is to start believing in their ideas and methods when things stop going well; we could even have Reimu thank Miko for indirectly helping her figure things out, as a callback to the ending of Symposium of Post-Mysticism when Miko found out about Reimu's true desire of being able to keep her friends and not having to fight youkai in order to keep Gensoukyou safe and in balance.
Maybe, Seiga Kaku. She taught Miko about Taoism in the first place, even though Seiga has always had her own plans and machinations going.
Maybe, someone from the Outside World. Ideally, this would be Sumireko Usami: not only is she the one human from the Outside World who still spends most of her time there, but I imagine she has quite a few strong opinions about current-day society in general and Japanese society in particular.
Maybe, Saki Kurokoma. In this context, it'd be a fantastic time for a Miko & Saki reunion.
At the end, there could be an epic final battle where Miko, helped by all the people who already gave her emotional support, fights either against an actual Lunarian or against a dark/corrupted version of Miko/Shoutoku who's loyal to the Lunar Capital (or even created by its leaders). Given that Byakuren would be the most likely to be involved in direct combat alongside Miko, and also that a Miko + Byakuren team-up would probably be the most thematically fitting for this story, we could use this combined arrangement of "Emotional Skyscraper ~ Cosmic Mind" and "Shoutoku Legend ~ True Administrator" by "Kyo-Sensei" as the final boss theme: https://www.youtube.com/watch?v=qlmG7ypo_eA
