I think it’s possible Zoro is a buccaneer.
The key trait of the buccaneers is their willpower and ability to push on in near death scenarios. Kuma is doing it now and it’s very likely how WB was able to fight so hard in MF.
Zoro’s endurance is near unparalleled. There are many characters we know of that have better durability than Zoro. But tbh no other characters bar WB and Kuma have shown endurance near that of Zoro’s. Where they can do something crazy like scar Kaido with their bones broken.
Even Luffy couldn’t endure ragnaraku, which pales in comparison to hakai.
Honestly Zoro just training hard or having high Will doesn’t explain his endurance, as even his captain cannot replicate his endurance feats. Hence imo it has to be some special ability.
Buccaneer powers would make a lot of sense. It seems as if Buccaneers are the servants of Nika. Kuma is the hand of liberation, he himself isn’t a D but he served Dragon and helped Luffy. WB himself wasn’t a D, but he devoted himself to raising Ace to be the new PK.
Zoro has devoted himself to helping Luffy become PK. If Luffy is the warrior of liberation, it only makes sense for Zoro to be the “sword of liberation”, being Luffy’s RHM.