I really doubt Ryuma is connected to the Void Century. From what we know, he fought against the fleets of the World Government, which we know didn't exist in it's current form during the events of the Void Century. Some magazine info suggests that he was alive 400 years ago.
And if he was alive during the events of the Void Century, why would he be buried in New Wano, not the Old Wano underneath the ocean?
Realistically, some Kozuki ancestor was the most prominent Wano citizen during the Void Century.
I'm not downplaying Ryuma or looking at this from a powerscaling perspective, it just seems the most logical to me.