Shiryu
a bum literally too weak, he will be lucky to even be in Zoro's last 4 opponents, mere great blade btw
Mihawk
can't be final opponent, after becoming WSS, Zoro HAS to be introduced as WSS, fight in a war as WSS and have a bounty of WSS.
This would be like Goofy becoming PK and never seeing his PK bounty and coming into a war to fight as a PK
Not to mention it logistically wouldn't make sense to have them fight last, Mihawk has been put in CG for a reason and that's to fight him for the one piece, mihawk ain't leaving CG for no reason and zoro ain't having some special last fight with mihawk in the series
Venus
He can be final opponent if Oda actually retcons gorosei from being punching bags by literal fodder and gives him a character besides being just generic gorosei
But the bum so far is too weak right now
Also he does not fall into category of "being 2nd strongest in villain the group" for Zoro's opponents since Imu, Peter, Garling and Topman all are possibly stronger than him, so it would make him 3rd strongest AT BEST
REAL POSSIBLE FINAL OPPONENTS:
Aramaki
He's Mihawk but better
+supreme black blade swordsman
+top tier logia
+can be 2nd strongest marine after Akainu, thus falling into zoro's opponent archetype
So he would fit for Zoro who just beat Mihawk and now has to prove against a black blade wielder who also uses strongest devil fruit category as well
Kizaru
+top tier logia
+can be 2nd strongest marine after Akainu, thus falling into zoro's opponent archetype
+has a sword
+so far fought the Vice Captains of the crew (Rayleigh, Marco)
+can be some kind of Sanji rivalry gag by him fighting Kizaru who is most like sanji while sanji fights Aramaki who is most like Zoro, thus they fight each other.
Fujitora
+already had an encounter and history with him
+supreme blade swordsman
+can be 2nd strongest marine after Akainu, thus falling into zoro's opponent archetype
+after Mihawk who has highest CoA mastery, he may fight Fuji who has highest CoO mastery