There have been several of those theories based on rebirth concept of Buddhism. Most of the theories couldnt connect Zoro's progression with the realms very well in my opinion.
The best explanation I found was probably posted in the Zoro FC of OJ by an user with "alastor" id if I remember correctly.
So basically there are 5/6 realms mainly and Alastor was connecting them with all the spirits Zoro displayed so far, for example:
Hungry Ghost realm:
animal realm:
asura realm:
I cant remember what he posted for the hell realm, and he speculated that Zoro hasnt reached the God realm yet