I understand that at some point Mihwak might have been planned to be his final opponent because he truly seemed like his final opponent but I feel that with the introduction of the WG being actually strong and Mihawk being part of CG this doesn't make sense anymore
To begin with, it's clear to me that the final opponent for Luffy is gonna be Imu, he is the man who defeated Joyboy and he is a fitting final boss for Luffy. About Sanji, I feel a Gorosei is much more fitting final opponent for Sanji, all Crocodile has is an strong fruit and Luffy already beat him, in comparison the Gorosei are inmortal, have Conqueror's Haki, can get omen boosted and a ancient weapon. Now, logically if the final opponent of Luffy and Sanji are Imu and an Elder then Zoro's final opponent should be an Elder too especially when Oda loves pushing the idea of Zoro=Sanji, did it Ennies Lobby, Wano and now in Elbaph so odds are he will do it again in the final battle.
A much more likely outcome is that CG will fight Strawhat before the final war where Zoro will defeat Mihawk and become WSS, thus ending his individual character arc and after that we will go to fight Imu and his underlyings and ending the overall story.
Now a common argument is that Zoro will become WSS in the epilogue but this makes no sense because Zoro isn't the MC and in nearly 30 years Zoro has never gotten the last fight of the arc, 'tis always Luffy's batle which is the climax of the arc and for the final war is would make the most sense if Luffy's final fight was the end of the story.
So to end this I think it doesn't make sense anymore for Mihawk to be Zoro's final opponent because the World Goverment are being setup as the final villains for the strawhats, for Luffy is Imu, for Sanji is an Elder so for Zoro is logically should be an Elder as well.