The Soul-Cutting Blade
But Why ?
Roronoa's final opponent before Luffy becomes Pirate King is most likely going to be Shiryu of the Rain.
Who has added to his arsenal the power of the "Clear Clear Fruit".
For now, the fruit seems to provide a similar ability to that of the Raid Suit, but it's possible that with enough mastery it can be pushed a significant step further.
When King had Sanji pinned with his beak burried deep into his belly he tells him that he should "disappear and escape", and Sanji clarifies that "it's just a visual disappearance".
What if the "Clear Clear" Fruit can allow for "true disappearance" ? As in, one can exist in a place, without a phyisical form ?
If that was possible, there's no way Zoro could cut something that does not have a physical body, unless..,
But How ?
Zoro's comment on how he first experienced the Breath of All Things was as follows
"This is the boundary between Life and Death"
At the boundary between Life and Death is where he could first experience the BOAT. Basically, the closest point to one's soul being separated from his body.
So maybe BOAT requires detachement from one's physical existence, in order to gain a perception of the world that would otherwise not be possible.
Another piece of the puzzle could be Kinemon's Fire Cutting Technique.
Back in WCI, we've seen Luffy attack Prometheus with Haki
And while he could alter the shape of his body, it could do no damage.
Kinemon's technique however actually hurt Prometheus
He screams in pain, which did not happen when Luffy attacked him.
So then Kinemon's technique can do more than carving a hole in the fire, it cuts at something more than it's appearance.
It's also interesting that Zoro was first made to use this against Big Mom's Fire Homie. The same Homie who was noted to be hurt for the first time ever by a fellow swordsman who can interact with Souls because of his DF ability.
So as a step towards achieving his goal of being able to cut anything, Zoro "steals" the ability to cut fire and hurts a Soul infused creation in the process.
The final step then when all the material world has been exhausted, could be the ability to cut directly at one's Soul.
"Then the Lord God formed man of dust from the ground, and breathed into his nostrils the breath of life; and man became a living being."
The Breath of living beings could be their Soul. When Zoro learns to perceive the "Breath" of his opponents beyond the material they are made of, he will be able to perceive their Soul, and maybe even eventually damage it.
So if his final opponent before Raftel has an ability that allows him to become immaterial, Zoro would still be able to perceive his "Breath", feel the presence of his Soul, and maybe even attack his Soul directly with a blade that can cut anything.
This could also tie in nicely with Luffy hearing the Voice of All Things. He can communicate directly with the Soul of living beings, and thus understand the Sea Kings or Zunisha.
