Alright, so this might be a bit crackpot but its a kinda interesting idea I had. By the way manga spoilers in case you clicked on this by accident or out of curiosity.

So, in the manga Vegapunk explains a theory he has about how devil fruits came into existence, talking about how it was people wills, dreams, imagination, etc. that brought forth devil fruits into reality. This idea was always a little hard for me to fully grasp how it’d work, now obviously there is likely more to devil fruits then this but Zoro’s Asura form might be a hint to how this works.

Zoro’s Asura is the very pinnacle of his ability as a swordsman. It allows him to increase his attack power by three times, by creating 6 more swords for him to use in combat. During Zoro’s very first usage of the Asura Style against Kaku, he used a technique called the “Demon Aura Nine Sword Style: Asura Ichibugin,” and this technique was quite incredible in itself.

Against Kaido, Zoro used a technique called Ashura Bakkei: Mōja no Tawamure, and this technique is one of Zoro’s very strongest to date. In application, it resembles Zoro’s Makyusen quite a lot, however, it is much more powerful.

Along with the power of the demon spirit of Asura, this technique also sees the application of high-level Armament Haki. At the same time, it also drew upon his Conqueror’s Haki. Thanks to that, Zoro’s power grew manifold when he fought Kaido and allowed him to scar the Yonko permanently.

Zoro’s Asura form has never been fully explained, the closest we got is that he uses conqueror’s haki while using it which really ain’t much.

However, Zoro’s Asura form could be a real time example of a devil fruit being willed into existence, like in the distant future of One Piece, long after Zoro dies from old age or something, there could be a “Mythical Zoan Hito Hito no Mi: Model Asura”.

Theory by TheLastAlbanian (https://www.reddit.com/user/TheLastAlbanian)