Yeah, I'm not really sure this switch to Seinen is such a good idea. That's if it's even legit.
Anyway, I've been thinking. Again.
Zoro's Asura technique is supposedly a manifestation of his spirit. Many people have already hypothesysed that it's a form of haki. So let's say it is. Let's say, that aside from knocking people out, predicting people movements, empowering attacks and negating devil fruit abilities, haki is also capable of... 'manifesting', taking form as an attack or technique, unique to every person.
For instance, Zoro is a swordsman, so Asura is a sword technique. This opens up a whole new world of possibilities in terms of techniques and fighting styles in One Piece, especially now that we're approaching the New World, where Haki seems to be more prevalent. It could also explain why some 'normal' characters are so strong, normal meaning non-Devil fruit users, or people who aren't a certain type, like a swordsman.
Imagine that in the New World, people with Haki are able to use really powerful and unique, specialised techniques based on their traits and characteristics. I can't think of any examples other than Zoro: a swordsman who's spirit manifests into him having more swords and using a really powerful sword technique.
Am I making sense? This is all just 'what if' talk, I'm not saying there's evidence to support me, but on the other hand, we know very little about Haki so maybe Asura does use haki, and maybe this is how it works.
Kinda like one of those "manifestation of one's true/inner power" that you sometimes see in manga and movies etc.