^ Well, 'Eyes of the True Sage' is technically a Doujutsu, since by being fueled with chakra (with natural energy added to the mix), it allows the user to see Natural Energy within and all over the environment. Thus technically, Naruto already obtained a doujutsu

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Not to mention that Senin Mode activates the body in a number of ways, you never know what it will activate in the future... and what it might activate in something that isn't human...
Of course we can't forgot how truly powerful the Nature Energy really is. It allows for phenomenal transformations and mutations, a CS user (which is based on a limited form of Senin Mode/Natural Energy) like Jyugo is an excellent example of it.
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