Honestly Zoro vs Mihawk may just be something they do on their own, something that doesn't happen while Luffy and Sanji are also fighting someone, because there's no valid opponents for Luffy and Sanji that'd align with Zoro's duel with Mihawk
If Luffy fights BB, it's very likely Zoro fights Shiryu.
If they're fighting against the navy, Zoro fights an Admiral like Issho or Kizaru if Kizaru survives this arc.
I always thought that, logically, the pecking order would be:
Shiryu > an Admiral or Gorosei (possibly) > Mihawk
But no matter what I feel that Shiryu is guaranteed to be before Mihawk as he's the fight Zoro needs to further his Observation Haki, just like King was a fight based around helping him control Enma better and unlock ACoC after he already unlocked CoC against Kaidou, Shiryu will follow that pattern of Zoro's serious opponents (so this excludes people like Pica and Lucci obviously) pushing him to get a new ability.
And since BB's likely going to be their last fight right before Laugh Tale (or it may even be ON Laugh Tale), this would mean Mihawk comes after the Shiryu fight.
So like I said, unless they then fight the navy or who ever's left over in the World Government, Zoro will probably be the only one with a fight when he fights Mihawk.