Watsuki says he'd like to know why Zoro has become one-eyed, but ODA DOESN'T ANSWER THIS QUESTION...
Now many fans said it's just an aesthetic upgrade,
Zoro clearly doesn't use an eyepatch.
Zoro's eye has been shut since pre-time skip, and nobody in the crew batted an eye at that.
So if doing this is something kind of training or a power-up. What could it be?
We have swordsman who have scars over their eyes.
I don't know if Fujitora can be counted or not, because we know his injury is self-harm.
Also, Xebec's mark over his eyes is probably A tattoo.