We know at least partly what Zoro is lacking though.
A stronger conqueror's haki.
This is what Zoro said when he awakened advanced conqueror's haki and entered KoH mode
"So Oden was giving you this much haki easily, huh?"
The implication here was that, when Zoro was overusing advanced conqueror's/armament in KoH mode, he was matching the haki that Oden was using for an extended duration of time.
Zoro could maintain this haki output only for like ~ 10 mins, while Oden can probably go on for a long duration of time.
Zoro would become a solid top tier swordsman, when he can maintain that level of haki output for an extended duration of time. Oden was someone that Kaido had in his "top 5", along with Shanks and 3 dead legends.
A "sky split" moment would probably signify this as well.
From there, I think just black blade is remaining as a powerup, which he might attain when fighting Mihawk.
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Yes, how these characters reach that level is important.
What is unfortunate however is that Oda simply doesn't care.
How did Luffy go from being like low-mid diff'd post 1010, to sky splitter after just resting and eating up?
We don't know. LOda just made him that strong. A dude who didn't know advanced conqueror's haki before 1010, became a "sky splitter" conqueror by 1026 by just getting beaten up and resting/eating. No explanation, nothing. He just powered up, that's it.
So, Zoro probably will become a sky splitter randomly.........just cause Oda wants him to be