Naruto had Sasuke, Gon had Killua, Goku had Vegeta (in Super). The Deuteragonist is described as the character 2nd in importance to the Protagonist. This applies to the aforementioned examples.

Sasuke is easily the 2nd most important character to the main story of Naruto. In regards to The Hunfer Exam, Heavens Arena, Greed Island, and Chimera Ant Arc, Killua is 2nd to only Gon. And he even becomes the Main Character for a bit. And for all of Super, Vegeta is inarguably the #2.

Zoro…just isn’t. He’s not more important than someone like Sanji or Nami or any other Straw Hat.

Really, in One Piece, you could argue that the Deuteragonist switches arc by arc. And if you wanna say Zoro was the Deuteragonist of East Blue then fine.But Alabasta had Vivi, Water 7/Enies Lobby had Robin/Usopp, Marineford had Ace, Punk Hazard and Dressrosa had Law, and Whole Cake had Sanji.

But even then, none of them ever remained as the Deuteragonist. Because One Piece doesn’t have one. Not every story does, and people seem to think that every story needs to have one.