I don't think i ever adressed this theory on Zoro's eye, so I figured why not share it and see what you guys' opinions are.

1st of all, I always figured zoro's nickname would eventually change, since he obviously isn't a pirate-hunter.
We, the readers, know he never really was one in the first place, but the government didn't know that.
Since he is now a pirate, that nickname is becomming outdated.

Before i begin, I think it is worthy to mention that the whole eye-matter has been left completely open.
If it was nothing more than a simple scar/wound, I believe oda would have resolved it by now (or even right after the timeskip).

As for the theory:
-During the course of the adventures, we've always seen quite a few displays of demonic aura manifesting in zoro and his techniques.
-Now it's safe to assume those manifestations were a product of Zoro's Spirit and on rare occasions, it sometimes seemed to be reflected into his left eye, making it glow red, before the timeskip.
Spoiler!

-We now know that Haki aswell, originates from an individual's spirit. And it's safe to assume Zoro did a lot of haki-training during his 2-year training.

I deem it very plausible, that near the end of his 2-year training, during a very intense sparring between him & Mihawk,
in a moment of extreme frustration and/or duress, the manifestation of zoro's demonic aura combined with a big burst of haki, his spirit somehow merged both, causing an anomaly which permanently ingrained itself into Zoro's left eye, leaving him with a "Demonic eye".
(It couls just as well have happened during another event, parhaps one related to his bounty-increase, idk)

Resulting effect: A constant burst of incredible power, speed & agility.

As for the theory for the scar itself:
Perhaps it caused Zoro to lose control, making him heavily assaulting Mihawk -perona and humandrills or others?-,
forcing Mihawk to cut it. (not permanently damaging it, but enough to shut it and make zoro regain his senses)
I can immagine it drained quite a bit of Zoro's stamina (for now), though he'll eventually controle it and get used to it in the future.

Which brings me to my conclusion that in the future, if they were to finally change Zoro's nickname, it'll be something along the lines of 'Demon eye Zoro'.
Pehaps there may even come a time when the same thing will happen to Zoro's right eye during a very important battle?
-> Demon eyes Zoro

Spoiler!


This... or it's just a wound and he lost his eye.