Introduction
Many people seem to believe that it will be someone else. I've heard speculations that Shiryu will kill Mihawk and Zoro will then fight Shiryu instead. Or that Zoro's final opponent will be the Gandhi Gorosei.
I always found such speculations very lacking, but recent events have caused me to conclude with high confidence that the opponent that Zoro defeats to become the WSS will indeed be Mihawk.
The Oath
Mihawk says that no matter how long it may take he will wait for Zoro at the top and he challenges Zoro to surpass him:
(The title of the chapter is conveniently named "Oath").
Zoro promises Luffy that henceforth, until the day he once again challenges Mihawk, he will never lose:
Zoro's Desire to Confront Mihawk
After he made the oath, Zoro's desire to confront Mihawk had been repeatedly referenced during both pre timeskip and post timeskip.
Vs Cabaji
Vs Hatchan
Vs Kuma
Begging Mihawk
Vs Pica
Finally: Vs King
Conclusions
Zoro's desire to confront Mihawk was a strong underlying theme for several of Zoroโs major character moments in the series and in many of his fights. It's a battle that is 20 years in the making and which Oda continues to reinforce as time passes.
Even during the King fight (perhaps Zoro's biggest fight since Mihawk), Zoro had two epiphanies connected to his desire to challenge Mihawk again.
Zoro's ascension to the King of Hell is a necessity so that he can challenge Mihawk again.
Nearly a 1,000 chapters after Zoro made that oath, Oda directly references it through a flashback. This is an unequivocal display that:
To go through all this development for Zoro only to deny him a second confrontation with Mihawk wouldn't just be a massive betrayal of the fanbase that he led on for 20 years, it would also be a gross disservice to Zoro's character.
I don't know how the fight would occur, but regardless of whatever justification Oda has to invent to tie it into the plot of the story, the fight will happen.
At this point, Zoro simply must fight Mihawk again.
@Garp the Fist, @Nidai_Kitetsu.
Many people seem to believe that it will be someone else. I've heard speculations that Shiryu will kill Mihawk and Zoro will then fight Shiryu instead. Or that Zoro's final opponent will be the Gandhi Gorosei.
I always found such speculations very lacking, but recent events have caused me to conclude with high confidence that the opponent that Zoro defeats to become the WSS will indeed be Mihawk.
The Oath
Mihawk says that no matter how long it may take he will wait for Zoro at the top and he challenges Zoro to surpass him:



(The title of the chapter is conveniently named "Oath").
Zoro promises Luffy that henceforth, until the day he once again challenges Mihawk, he will never lose:



Zoro's Desire to Confront Mihawk
After he made the oath, Zoro's desire to confront Mihawk had been repeatedly referenced during both pre timeskip and post timeskip.
Vs Cabaji
Even before his duel with Mihawk (and subsequent oath), Zoro stated that he cannot lose "not even once" to anyone who calls themselves a swordsman:
During the battle, Cabaji plays dirty and repeatedly targets Zoro's wound from Buggy. Zoro gets angry proceeds to deliberately stab the wound and grievously aggravate his injury. Cabaji thought that he did it to protect his reputation by giving himself a convenient excuse.
Zoro counters that if he could not win with such light injuries he may as well give up on his dream immediately:
Zoro then proceeds to declare his dream:

During the battle, Cabaji plays dirty and repeatedly targets Zoro's wound from Buggy. Zoro gets angry proceeds to deliberately stab the wound and grievously aggravate his injury. Cabaji thought that he did it to protect his reputation by giving himself a convenient excuse.
Zoro counters that if he could not win with such light injuries he may as well give up on his dream immediately:

Zoro then proceeds to declare his dream:

Vs Hatchan
Zoro tells Hatchan that there was someone that he must meet again, and until that day no one can take his life away:
An alternate translation by Stephen (the current Viz translator):
Zoro declares that he cannot die until he once again challenges Mihawk.
Later on in the fight, this sentiment is reiterated. Zoro refuses to succumb to his injuries as he must live to face Mihawk again:
Here's a nice collage that captures just the relevant panels:

An alternate translation by Stephen (the current Viz translator):
Stephen said:
Zoro: "Six Sword Style"? What a joke.
What's so great about that?!
Let me tell you something, Octopus!!
There's a man I have to meet... And until I find him again,
Not even DEATH HIMSELF will take my life!!!
Yosaku, Johnny, lend me your swords...!!
What's so great about that?!
Let me tell you something, Octopus!!
There's a man I have to meet... And until I find him again,
Not even DEATH HIMSELF will take my life!!!
Yosaku, Johnny, lend me your swords...!!
Later on in the fight, this sentiment is reiterated. Zoro refuses to succumb to his injuries as he must live to face Mihawk again:


Here's a nice collage that captures just the relevant panels:

Vs Kuma
When trying to convince Kuma to accept his head in exchange for Luffy's, he reiterates his ambition:

Begging Mihawk
Even when Zoro seeks Mihawk's tutelage, he reiterates that he desires to surpass him:
I think this is a strong refutation to any claims that Zoro will not fight Mihawk because Mihawk trained him. Zoro sought training under the pretext of one day surpassing Mihawk.

I think this is a strong refutation to any claims that Zoro will not fight Mihawk because Mihawk trained him. Zoro sought training under the pretext of one day surpassing Mihawk.
Vs Pica
After he bisects Pica's Golem with 1080 Pound Cannon, Zoro chastises Pica for looking down on him and once again declares his dream:
Then concludes by saying that he's on a much higher level than Pica.

Then concludes by saying that he's on a much higher level than Pica.
Finally: Vs King
During his fight with King Zoro references his promise to become the WSS when he awakens Haoshoku coating:
After he defeats King, Zoro recalls his promise to never lose again until he faces Mihawk, and states that if it is what it takes, he'll become the "King of Hell":
Zoro needs to become the King of Hell in order to challenge Mihawk.

After he defeats King, Zoro recalls his promise to never lose again until he faces Mihawk, and states that if it is what it takes, he'll become the "King of Hell":

Zoro needs to become the King of Hell in order to challenge Mihawk.
Conclusions
Zoro's desire to confront Mihawk was a strong underlying theme for several of Zoroโs major character moments in the series and in many of his fights. It's a battle that is 20 years in the making and which Oda continues to reinforce as time passes.
Even during the King fight (perhaps Zoro's biggest fight since Mihawk), Zoro had two epiphanies connected to his desire to challenge Mihawk again.
Zoro's ascension to the King of Hell is a necessity so that he can challenge Mihawk again.
Nearly a 1,000 chapters after Zoro made that oath, Oda directly references it through a flashback. This is an unequivocal display that:
- No, Oda did not forget.
- Yes, the oath is still very important.
- Yes, Zoro still aspires to Mihawk.
- And yes, they will indeed fight again.
To go through all this development for Zoro only to deny him a second confrontation with Mihawk wouldn't just be a massive betrayal of the fanbase that he led on for 20 years, it would also be a gross disservice to Zoro's character.
I don't know how the fight would occur, but regardless of whatever justification Oda has to invent to tie it into the plot of the story, the fight will happen.
At this point, Zoro simply must fight Mihawk again.
@Garp the Fist, @Nidai_Kitetsu.
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