Zoro's opponent will be the one that helps Zoro grow stronger in a way he needs to grow. The strength of Zoro's opponent isn't important. What is relevant is how they help him grow by exposing an area he is weak in, compared to the rest of his skills and ability, or he needs to grow in.
At this time, Zoro won't benefit much from fighting and beating Kuzan. Kuzan uses ice powers and haki. His ice DF powers appears to be his strongest ability. Zoro's powerful CoA haki , ACoC, cutting power/skill, monsterous strength, and fire/hellfire power should allow him to defeat Kuzan without need anything more or new.
Shiryu could make Zoro struggle if his DF awakening powers allows him to become harder to detect or complete erase his presence. We don't know much about his haki strength or swordsmanship but I doubt it's stronger than Zoro's.