Here is what I think the road of Zoro will look like for the WSS title from now on:
1.)The fight against Kaido. Getting the recognition of Kaido as a monstrous samurai, accomplishing something huge, his name starts to truly get some damn big recognition in the One Piece world. Let's remember that he wants his name to reach the heavens. He will leave Wano as a different monster.
2.)The second big event for him will be the fight against Shiryu. Now, Shiryu has the invisible fruit which might not mean much by itself since observation haki exists to detect your presence. However, there is a video from Mr Morj where he explains the possibility of a different application of haki that could exist: concealing your presence to the haki of others. He brings up some very convincing examples in the story where this application of haki might have been used. Now, under this assumption, Shiryu would have this type of haki AND the invisible fruit, effectively making invisible BOTH visually and to your haki as well. Truly undetectable. While of course he would also have the lethality of a top swordsman.
This could be fundamental in order for Zoro to improve his senses/make him develop some kind of particular observation/perception. After all, he is going to have to fight a man whose epithet is "Hawkeye" and , originally, "The Clairvoyant", which does suggest that armament haki is not really the only thing where he excels at.
I would suggest you to watch this Mr Morj video. Search something like "The new haki of Sanji in Wano".
3.)Then he will face Kizaru, a top tier. Throughout the story, Kizaru has always been matched up against first mates and vice captains. The examples are clear : Kizaru and Rayleigh, Kizaru and Marco, Kizaru and Benn Beckman. Kizaru has also kicked Zoro's ass in Sabaody and although Zoro might not hold a big grudge against him for that event, it's still something that I feel must be brought up. The reason for why Fuji is extremely unlikely to me is that he has already opposed the ideologies of the WG in way or the other. His actions are also often inconsistent with the way of thinking of the WG and he has already some major disagreements with the higher ups. Dresrossa was another big eye opener for him. To me, Fujitora seems perfectly set up as someone who will inevitably oppose the WG, probably in the final war.
Kizaru hasn't shown any sign of being against the current order whatsoever. And he is shown to be very close to Akainu, who has had major disagreements with Fuji. Kizaru vs Zoro is WAY more likely in my opinion.
4.)Mihawk. Yeah, I think Kizaru is a top tier just like him but I think that there is still some gap between them. Mihawk is a likely candidate for the third position in my list of the strongest current characters, after Dragon and Akainu. Kizaru would be at number seven, probably.
This of course will be his greatest challenge and will solidify him as the WSS.