Even better explanation.
Zoro was just some kid from East Blue who thought it was really cool being strong with swords.
Then he grew up and really learned to appreciate swordsmanship as whole.
He then creates a bond with someone else that drove him to improve day after day, but once that person died, he vowed to keep their promise to reach the top for both of their sakes.
It's basically what we already have.
All Oda needs to do is....nothing. actually nothing. really doesn't need to add anything. It works already. It's a good backstory.
If we learn about Zoro's lineage, we are gonna learn that Zoros parents died and Zoro had to eat his mother's flesh in order to survive or something saddest and twisted shit like that in order to top whatever else we got already.
