It's how Devil Fruits are born. Zoro's desire to become the strongest resulted in the manifestation of the demon god Ashura. Once he dies, a lucky person might be able to find the Devil Fruit it was reborn into, carrying Zoro's will in it.
Asura exists though, just like Nika existed once,
BB's Flag clearly indicates that Three Headed Demon is not Unheard of,
Also, one of Explanations of why Asuras are Three in Hinduism is that They symbolize Three Primary Ego-Driven Tendencies, which are Desire, Wrath & Cupidity, which can easily be associated with Fame, Power & Wealth,
I mean Zoro's Legacy isn't just becoming Strongest, but also having his Name reach Heavens, and leaving behind Best Sword(s),
So as an Asura, Zoro isn't Challenging OP God in not just Power, but also Fame & Wealth (Which in One Piece refers to what you Treasure, and in Zoro's Case it's his Sword),
Also, the Idea is that Zoro's Willpower requires "Three" Bodies to carry it, so if He was to give birth to a DF, He would create Three, not just One, similar to how the Guy who happens to wave an Asura Flag can eat more than One DF, and similar to how Oda refused to give Zoro a DF in SBS, and said He would give it to his Sword instead, implying that Zoro deserves Three DFs, not just One,
But Overall, seeing how DFs are born from Desires, and according to People, They defined Roger's Treasure (Who from their Perspective discovered everything this World has to Offer) as Wealth, Fame & Power (Meaning They believe anything in this World belongs to one of these Three), then Asura being Embodiment of such Desires indicate that it's not just a DF, it's Origin of DFs!
If Nika is Inspired from Vicki The Viking (Meaning He was Elbaph Viking who looked Smallest & Weakest, but Solved all Problems with his Imagination/Ideas), then God of Elbaph during Nika's Time (Who is probably what They attribute Spear of Elbaph to, meaning He was their Odin) was Strongest Warrior and inspired from Leader of Vikings in that Cartoon,
Do you know what's in common between him & Odin & Zoro? They are One-Eyed!
So i'm not saying you're wrong about Zoro manifesting a DF, but Zoro is much bigger deal than just that,
He apparently Inherited The Will of "Strongest Warrior" (His Name was Probably Elbaph)
Edit: Ryuma who is also part of Zoro's Line of Inherited Will, was also One-Eyed & described to carry Greatest Warrior Will.