I'm going to breakdown using narrative reasoning the entire story around Zoro's eye.
1. The eye isn't blind.
Let's get this out of the way first. Zoro did not lose his eye during the time skip. Narratively it doesn't add up that every strawhat came back better than ever and Zoro's the ONLY ONE who gets a ridiculous handicap like this. Zoro's not that special that for some reason he needs to be permanently nerfed for the rest of the story like this. Also his training isn't in any way shown to be more dangerous than anyone else's. Plus narratively, losing body parts isn't even a sign of danger. Shanks losing his arm for example isn't a sign that his encounter with the Lord of the coast is the most dangerous thing he's ever faced. Losing body parts such as Kyros losing his leg exist entirely for emotional weight and given no one's two year training has been given any time, it's doubtful there's an emotional story brewing in Zoro's training. Lastly the scar above Zoro's eye doesn't mean he lost it. Rayleigh and Shanks exist for example but also Zoro has scars on his legs and chest and never lost any organs despite those scars.
Counter
Meta-narratively, Zoro's post time skip design would change quite drastically if one day he opens his eye. Like all the marketing images and toys and whatnot would have to re-incorporate open eye Zoro which is something very hard to do.
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The meta-narrative problem of his image only matters if Zoro PERMANENTLY opens his eye. By that I mean if Zoro decides that after opening his eye, he never needs to go back to keeping it closed ever again. It's more likely that this won't happen. Zoro will open his eye for specific situations and close it back up again afterwards. That way the closed eye Zoro remains the default brand Zoro.
2. He's closing the eye as training.
There's two possibilities if the eye isn't blind; Zoro is willingly keeping the eye closed or some external person or power is forcing him to keep his eye closed. It's ridiculous to assume Zoro in 400 chapters would not tell his comrades that he's under some foreign power that's nerfing his ability and even more ridiculous that Zoro, maybe, is aware that he is under influence of some kind making him close his eye. Therefore it's more likely that Zoro is keeping his eye closed himself. And why? To train of course. We have seen Zoro nerf himself when he's training such as him meditating half naked under a freezing waterfall in Drum Island. And speaking of meditation, closing your eye interminably can be considered a form of meditation akin to the vow of silence among Buddhist monks, at least in Battle Shonen logic.
Counter
It's unlikely that Zoro would nerf himself in the new world. He's been very vocal about taking the new world seriously and yet he's nerfing himself. Seems uncharacteristically contradictory.
Counter counter
The narrative has made sure that Zoro never goes all out to require 'un-nerfing' himself even in the new world. Zoro is very vocal indeed about taking things seriously but he's also able to gauge how much effort a given situation needs and clearly not much effort has been required of him so far. Even in situations when he's out matched like Fujitora, there's a chance that opening his eye would not have provided any better odds. By this I mean, Zoro opening his eye will result in a specific effect outside of depth perception and that specific effect might not have been what was required to overcome Fujitora to begin with therefore it would've been a waste of time to do it. Also, Zoro is nerfing himself but he's also training here. There's a specific purpose to this so it's not just a waste of time like the carelessnes Zoro was telling Luffy to avoid when he told him to take things seriously.
3. Opening the eye has a specific effect
Like I said, closing an eye has the general nerf of destroying depth perception and balance and so those are a given when Zoro opens the eye but that's narratively not enough. This event must come with a specific effect. Whatever it might be, it must also be limited so as to ensure Zoro closes his eye again after the effect is used because of reasons stated above regarding Zoro's meta-narrative design. And of course limits on effects in storytelling are what ensures that the effects are actually strong. It's only a trump card super move if it has limited time or range or number of uses or something.
Counter
There is no precedent for this sort of "specific effect" created from meditation of any kind in the story.
Counter counter
There is no precedent for A LOT of Zoro's abilities already. It's almost a theme at this point that there's not much set up or explanation for what Zoro can do and so him having this 'specific effect' that comes from a ridiculous meditation regiment isn't even out of theme for him.
So What actually happened in the two years?
First of all, Zoro did NOT fight Mihawk. Zoro learned his lesson in East Blue not to step up to Mihawk ever again until he was ready. Therefore not even a light sparring match happened between them. Narratively alone it's an entire anti-climactic mess if any form of this rematch happened off screen and a one page flashback will never fix that. At best, Mihawk demonstrated something in front of Zoro but they never clashed once. In the end, the scar over Zoro's eye could be self inflicted as we've seen Zoro have multiple self inflicted wounds or a third party yet to be revealed.
Also it's most likely Zoro closing his eye is entirely unconnected to Mihawk as well. We've established that Zoro's abilities have very little precedent and so Mihawk having some knowledge of this specific effect among Zoro's abilities is unlikely. At best it's possible Mihawk told Zoro to do a specific thing and then Zoro came up with the eye meditation as his own way of achieving that specific thing but entirely based on his own weird abilities. This adds some narrative cohesion to the rest of the Monster trio since they too at points STOP training with their teachers and start training themselves. This was mostly because they, Luffy having a df and Sanji disliking Okama, had different styles from what their teachers could teach so they trained themselves. And of course Zoro's style is very different from Mihawk's so it fits that at a point he continued with training himself.
Foreshadowing
This shows the pupils of Koshiro's dojo mimicing training in the three sword style. Oda chooses to have the kids mimic Zoro's closed eye here for some reason. The kids do not mimic Zoro's "looks" in any other way. They don't copy the three earrings or bandana. They just have three swords and the closed eye. Obviously the kids do not know of the significance of closing the eye if there is one but then Oda still chooses to add it to their training regiment as if it's actually part of the training in the three sword style.
LASTLY, is this a Chekhov's gun or not?
There was the possibility that this is entirely not a plot element at all. That it never once comes up how or why the eye is closed given how no one asks about it in the story. Similar to the pre time skip Sanji design where it wasn't clear what his other eye looked like. But Oda does eventually answer what Sanji's other eye looks like. Even though in the story no one asks for the answer so it's not a mystery in the story. Oda as the writer knows that people ask the question so he knows he has to answer. Usopp and Franky for example got explanations right away for the changes to their bodies. The deliberate omission of an explanation for Zoro is almost in itself drawing attention to it and setting up the Chekhov's gun.
But then what if the answer is a joke. All the narrative reasoning above could easily be thrown out for the sake of a joke. This is also unlikely because it involves combat which is a key factor in Zoro's character. If for example there was something different about Zoro's haramaki (the green belly warmer around his stomach) then that would be something worth paying off with a joke. Zoro losing a body part seemingly during combat and which body part will affect his combat going on is easily the most drastic change along side Usopp and Franky after the time skip. And again those two get explained with plausible reasons behind them.
P.s,
Is it a Sharingan?
No. The specific effect is probably not even Haki related. It's most likely to do with Zoro's myriad of random weird abilities. Meaning it's completely unpredictable what it will actually do. The closest thing to a good guess is the 'Divine eye' in Buddhism that allows one to see Truth from illusions. Whatever that means.
1. The eye isn't blind.
Let's get this out of the way first. Zoro did not lose his eye during the time skip. Narratively it doesn't add up that every strawhat came back better than ever and Zoro's the ONLY ONE who gets a ridiculous handicap like this. Zoro's not that special that for some reason he needs to be permanently nerfed for the rest of the story like this. Also his training isn't in any way shown to be more dangerous than anyone else's. Plus narratively, losing body parts isn't even a sign of danger. Shanks losing his arm for example isn't a sign that his encounter with the Lord of the coast is the most dangerous thing he's ever faced. Losing body parts such as Kyros losing his leg exist entirely for emotional weight and given no one's two year training has been given any time, it's doubtful there's an emotional story brewing in Zoro's training. Lastly the scar above Zoro's eye doesn't mean he lost it. Rayleigh and Shanks exist for example but also Zoro has scars on his legs and chest and never lost any organs despite those scars.
Counter
Meta-narratively, Zoro's post time skip design would change quite drastically if one day he opens his eye. Like all the marketing images and toys and whatnot would have to re-incorporate open eye Zoro which is something very hard to do.
Counter counter
The meta-narrative problem of his image only matters if Zoro PERMANENTLY opens his eye. By that I mean if Zoro decides that after opening his eye, he never needs to go back to keeping it closed ever again. It's more likely that this won't happen. Zoro will open his eye for specific situations and close it back up again afterwards. That way the closed eye Zoro remains the default brand Zoro.
2. He's closing the eye as training.
There's two possibilities if the eye isn't blind; Zoro is willingly keeping the eye closed or some external person or power is forcing him to keep his eye closed. It's ridiculous to assume Zoro in 400 chapters would not tell his comrades that he's under some foreign power that's nerfing his ability and even more ridiculous that Zoro, maybe, is aware that he is under influence of some kind making him close his eye. Therefore it's more likely that Zoro is keeping his eye closed himself. And why? To train of course. We have seen Zoro nerf himself when he's training such as him meditating half naked under a freezing waterfall in Drum Island. And speaking of meditation, closing your eye interminably can be considered a form of meditation akin to the vow of silence among Buddhist monks, at least in Battle Shonen logic.
Counter
It's unlikely that Zoro would nerf himself in the new world. He's been very vocal about taking the new world seriously and yet he's nerfing himself. Seems uncharacteristically contradictory.
Counter counter
The narrative has made sure that Zoro never goes all out to require 'un-nerfing' himself even in the new world. Zoro is very vocal indeed about taking things seriously but he's also able to gauge how much effort a given situation needs and clearly not much effort has been required of him so far. Even in situations when he's out matched like Fujitora, there's a chance that opening his eye would not have provided any better odds. By this I mean, Zoro opening his eye will result in a specific effect outside of depth perception and that specific effect might not have been what was required to overcome Fujitora to begin with therefore it would've been a waste of time to do it. Also, Zoro is nerfing himself but he's also training here. There's a specific purpose to this so it's not just a waste of time like the carelessnes Zoro was telling Luffy to avoid when he told him to take things seriously.
3. Opening the eye has a specific effect
Like I said, closing an eye has the general nerf of destroying depth perception and balance and so those are a given when Zoro opens the eye but that's narratively not enough. This event must come with a specific effect. Whatever it might be, it must also be limited so as to ensure Zoro closes his eye again after the effect is used because of reasons stated above regarding Zoro's meta-narrative design. And of course limits on effects in storytelling are what ensures that the effects are actually strong. It's only a trump card super move if it has limited time or range or number of uses or something.
Counter
There is no precedent for this sort of "specific effect" created from meditation of any kind in the story.
Counter counter
There is no precedent for A LOT of Zoro's abilities already. It's almost a theme at this point that there's not much set up or explanation for what Zoro can do and so him having this 'specific effect' that comes from a ridiculous meditation regiment isn't even out of theme for him.
So What actually happened in the two years?
First of all, Zoro did NOT fight Mihawk. Zoro learned his lesson in East Blue not to step up to Mihawk ever again until he was ready. Therefore not even a light sparring match happened between them. Narratively alone it's an entire anti-climactic mess if any form of this rematch happened off screen and a one page flashback will never fix that. At best, Mihawk demonstrated something in front of Zoro but they never clashed once. In the end, the scar over Zoro's eye could be self inflicted as we've seen Zoro have multiple self inflicted wounds or a third party yet to be revealed.
Also it's most likely Zoro closing his eye is entirely unconnected to Mihawk as well. We've established that Zoro's abilities have very little precedent and so Mihawk having some knowledge of this specific effect among Zoro's abilities is unlikely. At best it's possible Mihawk told Zoro to do a specific thing and then Zoro came up with the eye meditation as his own way of achieving that specific thing but entirely based on his own weird abilities. This adds some narrative cohesion to the rest of the Monster trio since they too at points STOP training with their teachers and start training themselves. This was mostly because they, Luffy having a df and Sanji disliking Okama, had different styles from what their teachers could teach so they trained themselves. And of course Zoro's style is very different from Mihawk's so it fits that at a point he continued with training himself.
Foreshadowing
This shows the pupils of Koshiro's dojo mimicing training in the three sword style. Oda chooses to have the kids mimic Zoro's closed eye here for some reason. The kids do not mimic Zoro's "looks" in any other way. They don't copy the three earrings or bandana. They just have three swords and the closed eye. Obviously the kids do not know of the significance of closing the eye if there is one but then Oda still chooses to add it to their training regiment as if it's actually part of the training in the three sword style.
LASTLY, is this a Chekhov's gun or not?
There was the possibility that this is entirely not a plot element at all. That it never once comes up how or why the eye is closed given how no one asks about it in the story. Similar to the pre time skip Sanji design where it wasn't clear what his other eye looked like. But Oda does eventually answer what Sanji's other eye looks like. Even though in the story no one asks for the answer so it's not a mystery in the story. Oda as the writer knows that people ask the question so he knows he has to answer. Usopp and Franky for example got explanations right away for the changes to their bodies. The deliberate omission of an explanation for Zoro is almost in itself drawing attention to it and setting up the Chekhov's gun.
But then what if the answer is a joke. All the narrative reasoning above could easily be thrown out for the sake of a joke. This is also unlikely because it involves combat which is a key factor in Zoro's character. If for example there was something different about Zoro's haramaki (the green belly warmer around his stomach) then that would be something worth paying off with a joke. Zoro losing a body part seemingly during combat and which body part will affect his combat going on is easily the most drastic change along side Usopp and Franky after the time skip. And again those two get explained with plausible reasons behind them.
P.s,
Is it a Sharingan?
No. The specific effect is probably not even Haki related. It's most likely to do with Zoro's myriad of random weird abilities. Meaning it's completely unpredictable what it will actually do. The closest thing to a good guess is the 'Divine eye' in Buddhism that allows one to see Truth from illusions. Whatever that means.
