I think in a future arc we will see Zoro defeat Shanks.
I think next arc will be Elbaf and the arc after that will be lodestar. Either The Straw Hats will encounter the red hair pirates next arc or next arc will just be a story arc like Zou and the arc after that- Lodestar will involve the Red Hair Pirates.
We know that Zoro always ends up repeating or maybe even one-upping what Luffy did the previous arc... or at least the previous major arc. Luffy barely defeated a YC1 last arc, and this arc we saw Zoro beat a YC1 decisively. Luffy's first bounty was 30 million and Zoro's first bounty was 60 million. Luffy's second bounty was 100 million and Zoro's second bounty was 120 million. Luffy's 3rd bounty was 300 million and Zoro's was 320 million. Zoro always ends up surpassing Luffy's previous feat. This arc Luffy will barely defeat a Yonko with a lot of help, which is why I think in the next major arc Zoro will beat a Yonko 1v1.
And the Yonko he will beat will be Shanks. Zoro has a lot of reason to fight Shanks. Firstly, Luffy and Shanks are friends and Shanks is a sort of father figure to Luffy so I can't imagine Luffy fighting and beating Shanks. Especially since Luffy will beat both Kaido and Blackbeard. Fighting 3 yonkos back to back is kind of repetitive and I can't see Oda doing that. Luffy has already had his Yonko fight and has sky split and done all that jazz. BB will be difficult for a different reason because of his DF and Luffy having to overcome it. Fighting Shanks after already beating Kaido just won't be that interesting.
Secondly, Shanks was the former rival of Zoro's goal- Mihawk. What better training for a Mihawk fight than his ex-rival ? After beating Shanks Zoro will definitely be ready for beating Mihawk. (Maybe with a fight against Aokiji or Shilliew in between to train his CoO). Zoro wants to be the world's strongest swordsman, therefore he would want to fight and beat a famous and powerful swordsman like Shanks. He has already unlocked AdCoC but a battle with Shanks will allow him to refine it and master it and at some point (maybe near the end of the fight) I can imagine Zoro and Shanks splitting the sky. Luffy has already had his moment with Kaido and now it'll be Zoro's turn to clash with a yonko and split the sky.
How will the fight start ? I see 2 possibilities. Firstly, Shanks and Zoro could just have a friendly sparring session which slowly gets more and more serious and becomes a full-on fight.
The second option is some sort of a davy back fight but more serious where one member of Luffy's crew goes up against a member of Shanks crew. Think a tournament style thing. I could see Usopp fighting and beating his father.
Now the question is who will Luffy fight ? I think the answer is obvious. Benn Beckmann ! We already have a vivre-card entry stating that Ben is almost as strong as Shanks. I think he'll be able to give Luffy a challenging and entertaining fight. Kaido is a bit of a brick and brute so I see Luffy beating him in a slugfest with Boundman. Beckmann, though not as strong as Kaido is probably a more intelligent, strategic type of fighter (he was called the most intelligent in East Blue after all). I think he'll be a high-speed gunslinger type character (though maybe less purely speed-focused than Roo who Sanji will fight and more IQ-based but not as IQ-based as Yasopp) I think Luffy will beat him in a high-speed, high-skill, long-ranged CoO-heavy fight using Snakeman.
Which will leave the final fight between two swordsmen. Mihawk's old and new rivals. I think Zoro will be weaker than Shanks at the beginning of the fight but grow and break his limits during the fight and master AdCoC (splitting the sky) and ultimately win.
I don't think he'll kill Shanks or anything but he'll win the duel somehow. Maybe he'll disarm Shanks or break his sword or just have Shanks admit defeat. (I can see it being very similar to Luffy vs Katakuri and in the end Shanks will realise that he is the man who will beat Mihawk and become the new WSS and fall down first).
This could also set the stage for Blackbeard killing Shanks or something like some other theories have predicted as Shanks and his crew will probably be injured and BB, ever the dirty fighter will use this chance. Though this could also not happen.
We've seen almost no fights between Zoro and another swordsman in the series so we really have to get one more before the Mihawk fight. One of the few such fights was against Ryuma and that happened in Thriller Bark, the next major arc after Enies Lobby and Wano seems to be the Enies Lobby of the Post-TS.
Zoro vs Shanks will be an awesome fight and makes so much sense on so many levels.
