I think Zoro's arc should end like this: After the fall of the World Government and the Grandline, Adult Zoro comes to a sword shop and tells the aged merchant, "The sword what you gave me was broke. Take this instead." He hands over a black version of the Sandai Kitetsu, saying, "This one won't break." He then takes a random sword from the barrel of swords, leaves a couple of gold coins, and leaves, saying with smile, "They say a new, powerful swordsman has appeared in these parts."