There are 3 strong swordsmen available under the Yonko this war; Kyoshiro, King, and Smoothie
Kyoshiro is more than likely going to turn on the Shogun. Assuming the Shogun was referring to him when he mentioned somebody being untrustworthy.
This leaves us with King or Smoothie. Out of the two, King is more than likely going to be the stronger one, unless Smoothie is some hidden card that has yet to reveal her full powers as the strongest sweet commander or something, which is highly unlikely. Making King the most viable opponent for Zoro out of the swordsmen before he goes against Kaido.