You can say that Zoro's opponent or even Luffy's next opponent will be Yonko level but you cannot say that they will for sure be stronger than Kaido. There are only a few people who could potentially be stronger than Kaido and that is due to power creeping by the author as the story progresses in a manga. The candidates are Dragon, Imu & Gorosei , Akainu, BB, Shanks and Mihawk.