Before Marco's arrival last chapter, Zoro was the most popular candidate for defeating King. The main reasons to believe Zoro would defeat King are:
The Normal Formula
In almost every arc, Luffy defeats the antagonist and Zoro fights the antagonist's strongest subordinate. The formula has been pretty robust both pre and post timeskip.
The only occasions that come to my mind as exceptions are:
- Romance Dawn: Zoro defeated the antagonist (Axe Hand Morgan) himself.
- Morgan did not have any clear strongest subordinate. As far as I can recall, none of his subordinates were named. So one might argue that this doesn't really count.
- Baratie: Zoro did not fight any of Don Krieg's men.
- Zoro was tken out of commission by Mihawk and was in no condition for further fights.
- Arlong Park: It is not clear that Hatchan was stronger than Kuroobi. In fact, Kuroobi's Vivre Card mentioned that he was initially intended to be the Vice Captain of the Fishman Pirates. No mention was made of this detail in the manga, so it didn't actually materialise. This is not a clear exception, but it is included for completion.
Even in arcs that seem like they may have been exceptions, Zoro still fought the antagonist's strongest subordinate:
- On Punk Hazard: Vergo was stronger than Monet (and Caesar), but he wasn't a subordinate of Caesar's. Caesar's strongest subordinate was Monet.
- Dressrosa: Pica was clearly the strongest of Doffy's executives in the arc. Even if Vergo was above Pica, Vergo did not feature as a character in the arc, so he wasn't eligible. Doffy's strongest subordinate was Pica.
- It's worth mentioning that even though Luffy had assistance in his Doffy fight, Zoro still defeated Pica by himself.
As can be seen above, the exceptions in which Zoro did not fight the antagonist's strongest subordinate, there were very extenuating circumstances. The formula is
very robust. We'll need strong reasons to doubt that it applies to a particular arc. If Luffy indeed fights Kaido (whether with assistance or not), our priors that Zoro would fight King should be very high.
King is a Swordsman
King appears to be a (very powerful) swordsman. His blade has quite the level of detail to it, and appears to be a meitou:
Compare the level of detail on King's swords to the level of detail shown on two O Wazamono blades (Enma and Ame no Habakiri):
Giving that King appears to wield a meitou, our priors that he's a highly skilled swordsman should be farely high. Even in the Oden flashback, King appeared to use a sword in combat:
In the entirety of Wano, Zoro has yet to have a full fledged fight with a swordsman of significance. The closest he has gotten to one was his clash with Denjiro, but that clash was unresolved. On priors, it would be very surprising if Zoro does not get any swordsman battle in the entirety of Wano. Even in arcs that didn't heavily feature swordsmen (e.g. Enies Lobby, Thriller Bark and Fishman Island), Zoro got a swordsman to face (Kaku, Ryuma and Hyouzou respectively). As a result, it would be surprising if in the arc that does feature swordsmen very prominently, Zoro doesn't get a swordsman opponent to face. That King also happens to be the antagonist's strongest subordinate (like all the aforementioned swordsmen) makes it more likely that King (in particular) is the swordsman that Zoro will face.
King Uses Fire Abilities
King's epithet is "The Wildfire":
He appears to be capable of using his flames in combat:
He may be capable of creating fires that last days:
Zoro has indicated interest in a fire sword and Kinemon's Fox Fire style.
A swordsman that uses fire based abilities and is also the antagonist's strongest subordinate seems to be the ideal opponent for Zoro.
Zoro's Role in Wano
So far, Zoro has been the second most prominent character in Wano. He's ahead of even Oden in panel time, and dwarfs the other alliance members bar Luffy. He's forged connections to a lot of characters that play important roles in the arc (connections that are normally forged by Luffy):
- He met Otama.
- In chapter 980, Zoro agreed with Luffy's decision to attack the Beast Pirates for wasting food due to also bearing witness to Tama's suffering.
- He befriended Yasuie.
- Following Yasuie's execution Zoro swore to avenge him. This is very important as Zoro has said that he dislikes fighting for revenge. Yasuie gave Zoro emotional involvement in the conflicts of the arc and a reason to fight beyond supporting Luffy.
- He saved Hiyori and bonded with her.
- Hiyori is Oden's daughter and appears to be a secondary arc princess. She was the one who contextualised the suffering of the people of Ebisu town at the hands of Orochi for Zoro, explaining to him the curse of SMILE.
- She cried into his arms and expressed a desire to kill Orochi with his own hands.
- She also bequeathed Enma to him.
- He attacked Orochi.
- Kyoshiro blocked his attack on Orochi, leading to a clash between Zoro and Kyoshiro. Orochi doesn't appear to be a fighter in his own right (he was helpless before Kyoshiro's attack), so if Zoro's vendetta on Orochi comes to fruition, it would involve defeating whoever is protecting Orochi.
- He swore not to forgive Orochi and to avenge Yasuie.
- He received Oden's sword Enma.
- Hitetsu also suggested that Zoro may surpass Oden with Enma by blackening the blade, a feat Oden was unable to accomplish.
Wano appears to be for Zoro what WCI has been for Sanji, and he's set up to take a very prominent role in the arc. Even in the recent chapters with Kidd's reappearance, Zoro has not looked at all inferior to him and still features quite heavily. I would wager that Zoro would have the second most prominent role from the alliance's side (with Kidd being third). It would be very surprising if Marco was to usurp Zoro's role in this arc.
BM couldn't react and do a thing against King