The Otsutsuki might be the unavoidable threat in the Naruto franchise solely due to the fact that they keep returning to Earth. The process started with Kaguya Otsutsuki and is currently ongoing with the speculated return of Isshiki Otsutsuki. With the world currently in peril, who could be the perfect warrior against this threat? To some fans' surprise, the warrior might be the protagonist himself.
The protagonist overpowered the Otsutsuki on two occasions, in the prequel and the sequel series. In the prequel, the protagonist overpowered Kaguya using Sage of the Six Paths chakra, and in the sequel, defeated Isshiki using Baryon Mode. So, 'hypothetically,' a combination of these two modes might have been a menace for the Otsutsuki clan if such a form existed.
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Naruto: Why a Sage of the Six Paths plus Baryon Mode might be a menace for the Otsutsuki clan

The Otsutsuki clan is a race of otherworldly beings who visit planets to devour them. These beings devour the planet's accumulated source of life, the chakra fruit. Since the Naruto series, these beings have been trying to devour Earth using its chakra fruits, but due to the acts of the shinobi, these otherworldly beings are yet to do so.
The first Otsutsuki clan members that infiltrated Earth were Kaguya and Isshiki Otsutsuki. After betraying the latter, the former spent a peaceful life with a human. However, due to her insatiable thirst for power, Kaguya couldn't hold back and even schemed to devour her own children. Fortunately, her children, Hagoromo and Hamura, outclassed her and saved Earth from destruction.

Hagoromo then created ninshu (ninjutsu) and divided his powers into nine-tailed beasts. Unfortunately, Kaguya couldn't rest and returned during the Fourth Great Ninja War. However, this time, the ones to stop her were Naruto Uzumaki and Sasuke. The former, using his Six Paths chakra and the latter using his Rinnegan, saved Earth from destruction yet again.
However, this wasn't the end of worries as Isshiki made his return soon after, looking for a proper vessel to be reborn inside. As fate had it, Isshiki also crossed paths with the same opponents, Naruto and Sasuke, and things were different this time. The protagonists had come of age, and using the usual tactics wasn't a choice anymore.

So, the protagonist, after consulting his tailed beast (Kurama), pulled out a new power-up, the Baryon Mode. Fortunately, this move managed to outclass Isshiki but came at the cost of Kurama's life. Coincidentally, every Otsutsuki (including Momoshiki) faced the protagonist and has been outclassed.
So, what if the protagonist used his most overpowered power-ups to create a single one? A combination of Baryon Mode and Sage of the Six Paths chakra might be the sweet point that might be the nightmare for Otsutsuki clan members, given how these two forms have outclassed two of the strongest Otsutsuki clan members.
Final thoughts
As hypothetical as this combination might be, it would be the strongest of them all, capable of overpowering any antagonist that dared to steal Earth from its inhabitants. Kurama kept Baryon Mode as a last resort, so if Hagoromo hadn't manifested during the Ninja War, things might have turned out differently.
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