My theory is that zoro is directly tied to ryuma via the toki toki no mi. Rather than him being ushimaru's son like a lot of people speculated before oda debunked it, Ushimaru is actually zoro's descendent.
I think zoro either is Ryuma, who was reverted into a child by Bonney's fruit(healing his injuries along the way), or Zoro is ryuma's son. In either case, I think he was sent to our timeline by Toki.
That is why Toki wanted to go to wano when she appeared. She was looking for Ryuma.