Gai-chin is seriously asking what plotline Zoro had in Wano...
How about coming full circle and returning the national treasure that he took from the corpse of Wano's sword god?
How about being his descendant and meeting the only remaining Shimotsuki in Wano and accompanying him on his adventure?
How about personally witnessing the horror of Kaido-Joker-Caesar business in Ebisu town?
How about being a companion of arc's princess and saving her several times?
How about being the one who trained the new Shogun of Wano and told him about Sunachi?
How about being the one who inherited the sword of Kozuki Oden as well as being in possession of the sword that his father forged?
How about the plotline of cutting up into pieces the toughest on Onigashima?
How about the plotline of saving the new generation from two Emperors?
And those are just those that happened while plenty were started and weren't finished, like going to Ryuma's grave and killing Kaido.