So I am well aware that this site is something of an echochamber for the most fervent zoro fans, but outside this small community, Zoro vs Law is something that is debated on by a majority of the forum. A significant amount of reddit users believe that Law>Zoro as Law is Luffy's rival, has a higher bounty, and defeated an emperor whereas Zoro simply defeated King.
Regardless of what your opinions are on Zoro vs Law, in order for Zoro to be the WSSM, Oda needs to do one of these things:
- directly and unequivocally show that Zoro> Law
- by having Zoro either defeat Law outright, defeat a much stronger opponent than Law's, or have Law state his inferiority
- This isn't just something like Zoro using asura and Law making a soy face. It has to be something so significant that even non-zoro fans accept Zoro is stronger. After all, Oda isn't writing One piece just for the sake of Zoroboys on worstgen.
- portray that Zoro isn't in competition with DF users like Law
Personally, I wouldn't be surprised if later down the road, Law switches sides and tries to steal Luffy's poneglyph maps and we get Zoro vs Law to give us a definitive answer. But overall, by the end of one piece, Zoro vs Law shouldn't be up for debate. Either Zoro>Law, Law isn't in competition for WSSM because of his DF, or Zoro's title isn't definite.