Just an FYI, antagonist does not mean EVIL or villain. Merely its someone who impedes the goals or progress of the protagonist.
Tbh, I'm surprised by the number of people that haven't at least tried to understand the deduction behind the whole Shanks being the last antagonist/battle, solely by virtue of Mihawk.
Mihawk is the only peak tier character without an associated group. The strawhats always fight an antagonist or enemy faction, something that ties each of those antagonists together. Arlong Pirates, Boroque Works, CP-9, Thriller Bark Pirates, Donquixote Pirates, Beast Pirates. Everything neatly tied together to deliver faction battles each arc.
Yet, Mihawk is just unaffiliated with any group, EXCEPT the Red Haired Pirates. It begs the question then: narratively, Luffy and crew will fight some final antagonistic group in this series. If Mihawk is meant to be Zoro's final fight, someone who is no longer a Shichibukai or associated with the WG, but an acquaintance with Shanks.
Shiryu has never been associated to be remotely Zoro's final goal in this series. Hell, Luffy doesn't even talk about Blackbeard when it comes to his goals. It's almost always about Shanks; meeting Shanks, fighting Shanks, surpassing Shanks.
Even this color spread this week sort of supports this idea. Shiryu is in fact BBs number 2. He is a swordsman, we can see that. Now, Ben Beckmann hasn't been implied to he a swordsman at all. Of course, this does not mean Zoro has to fight someone with a sword (though that's been consistent since the beginning of the series), but if you just logically assume:
- Mihawk is longer a Shichibukai
- The final war is coming
- Final War will involve major factions fighting
- Thus Mihawk probably will be part of some faction
- His own known association is the RHP
With this, Shanks and Mihawk, Luffy and Zoro's respective final HUMAN goals (i.e. surpassing these people), make it very easy to understand why the RHP may be the final battles in this series. Then you can have Sanji fight Beckman, because while he's Shanks' #2, Mihawk is Shanks' equal (to an extent I don't want to get into that argument...).
Just my thought process behind these 2 crews. I think Zoro's fights make it pretty telling who he may fight in the end. You can even see on the Marine/WG side, Fujitora and the Monk Gorosei truly fit the mold to be fights for Zoro as well.