This Theory is actually related to a Greek Mythology as we can see in the series most of the characters and abilities are somewhat based on the Greek Mythology.There are always lots and lots of arguments relating to the Zoro's Eye.But none of those theories satisfy us and hence I've come up with a different theory I found out by researching Greek Mythology.
I think he is actually based on Oden as we can find about it surfing through the internet and how he gave up his eye for power and that could be the same thing with Zoro.
This is well related to Zoro as he is always seem training harder and harder to become stronger. He even tells himself during the training that he needs to be more and more stronger considering himself as far too weak .
He is willing to sacrifice his life in order to save his Captain Monkey D Luffy. So, I think he could go to any of the limits of his body to gain a tremendous power so as to become the Strongest Swordsman even also for being a great help to Monkey D Luffy(captain) to make him the King Of Pirates.I think we will obviously see this in Wano Kingdom when he fights Jack and surely will be able to defeat him since we haven't seen Zoro using his full power since the timeskip.
Theory By - Jouvens Marc