For me, the fight should happen after the Final War.
For anybody that said Zoro won't fight Mihawk because they're mentor/student relationship, I mean y'all know that Anakin vs Obi-Wan did happened right?
For those who said Shiryu will kill Mihawk or Gandhi will be the final opponent, fair but then again, Shiryu won't let a fair 1v1 duel happens, plus the fact that Zoro has been fighting tricky swordsmen (Mr. 1, Kaku, Pica, King, etc.) seems kinda repetitive already. Zoro vs Mihawk would be like Zoro vs Zombie Ryuma where no tricks were involved, just straight up duel to the death. Now that's something I want to see.
The case for Gandhi, unless he gets Bonney to turn him back into his prime, can't see Zoro fighting an old fighter. He's probably a low top tier which I don't see EOS Zoro being a low top tier after beating Gandhi.