Not everyone fight the same
For example when he fought Braham in Skypiea, he had to learn Flying Slashes cuz his Opponent was a Ranged Fighter & Zoro couldn't get near, even though Braham wasn't as strong as Daz Bonez
King Fighting Style seems to give Zoro trouble defending, cuz King so far relied on Disarming him, Attacking non-stop to not leave room for reaction & strong pushing power which keeps Zoro on the edge, and he will soon add lot of Fire to it, not to mention that he flies
So it's not really that Zoro needs to Power Up, his AP is already strong enough to Mortally Damage anyone & give a Permanant Scar to Kaido, but he needs to find a way to handle King's Troublesome Fighting Style, this is why we will soon see something New from Zoro
And in my opinion, it should be related to Defense, Zoro won't tolerate allowing himself to get Disarmed, lost his Footing multiple times & almost fell, so he needs something that makes him an Unmovable Swordsman (This Need was being foreshadowed multiple times if you notice Zoro's previous Opponents, Fujitora with his Gravity, Pica who was moving the Ground, facing Hakai, clashing with Dragon in Punk Hazard ... etc). So now it's time that Zoro learns something that makes him handle this kind of situations.
I believe that Zoro will decide to Charge Up his Barrier CoA (Similar to when he stopped Hyouzou or when Rayleigh stopped Elephant or when Sentomaru fought Luffy ... etc), but Zoro will perform a Very Top Tier version of it (Similar to Three Admirals stopping WB's Attack in MF), i can imagine a Scene where King charges up with all his Power in Flames for one final attack, but Zoro creates a very Powerful Barrier that not only stops King's Attack but leaves him very damaged & open for another Attack.
The only thing i'm not sure about, is whether Zoro will perform this Feat using Asura or not