This is something between a theory and a speculation.
We all know that Zoro is currently 21 years old, and since he was young he lived in East Blue.
There might be a particular reason of why Shushui chose to get wielded by Zoro.
I know that there are many that have speculated the same ideas, but i'm here to put some new thoughts based on the latest SBS (vol.92) (sorry if anyone has already wrote one based on this)
[I believe that Zoro is Ryuma's ancestor, but Zoro was born in East Blue.]
Appearance: Roronoa Zoro and Ryuma.
At near same age, they look alike a lot. And we know that Oda likes to draw siblings similar when they are the same age, if it is for brothers, sisters or sons to parents or grandparents.
Oda's states: SBS vol.92 chapter 926 which states:
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"D: In Ch. 909, was the carpenter boss that appeared in Wano the same one that was introduced in the Volume 7 SBS, Minatoma-san? P.N. Yacchi
O: Yes he is!! Back then, he was fixing the broken door to the bar in Foosha Village. He was introduced as Carpenter Minatomo-san, exclaiming "Who did this!?". And now, he somehow ended up in the New World in the closed bordered country of Wano! Isn't that weird!? That's right, they actually aren't the same person! They have the same last name because they're related by blood. The fact of the matter is, a ship from Wano reached East Blue a few decades ago.
One of the descendants of someone on that ship is a person that everybody is familiar with...
This part might be featured later in the main story so I won't mention any more than that. It isn't going to be a major plot line, it will just be a minor story."
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Now, if we assume here that the descendant is Zoro and his parent arrived East Blue a few decades ago, there's a lot we can speculate, like:
ex1: They left Wano after Roger's death knowing things will turn bad, arriving in East Blue.
ex2: They were onboard Roger's ship, when Zoro's mother got pregnant she decided to stay in East Blue.
We can hardly speculate the exact case here.
There is also a possibility that Koushirou was aware of Zoro's lineage. Kuina's death, falling off the stairs is weird, but that for other theories.
So, Zoro should have an honored way to keep the Shushui, since it's that of a treasure and we've seen how people react to it. I believe that this is the most noble story for Zoro.
What do you guys think?