Between Kitetsu and Shuunsui, Zoro has his hands full with blades that don't want to listen to him. I think the problem would only get worse if he got another Kitetsu. This argues that Zoro must strengthen his already iron will, if he is to have complete mastery over his swords. Moving on to the topic of which swords Zoro might replace:
As for Wado, fuggedaboudit. That sword is a symbol of his oath to Kuina's memory. He's already told both Ippon-Matsu and Tashigi that he wouldn't give up that sword for anything. It also withstood a head-on duel with Mihawk, so I doubt it's going to shatter, rust, or get cut to pieces.
Shuunsui, his latest acquisition, was the property of a famous swordsman from centuries past. It also serves as a tie-in, of sorts, with Oda's one-shot comic "Monsters". Sort of a way for Zoro to become the spiritual successor to Ryuuma, 'The King'- World's Greatest Swordsman of His Generation. If it has been wielded by the Greatest before, perhaps it's time for the Greatest to wield it once more…
Sandai Kitetsu, the Third Generation Demon-Splitter, is a Wazamono. A classification shared by only fifty-whoops! 49 swords in total. Take a look at Whiskey Peak, or Enies Lobby, or Thriller Bark, and you'll see plenty of katana. It's doubtful however that all of these are Wazamono-Grade or higher. Sandai Kitetsu is plenty sharp- it cuts through stone without any special effort on Zoro's part, a fact he commented on when he first used it in battle. It may not be as sharp as its forebears, or as strong, but it is supposedly just as cursed. Since Zoro was willing to bet his luck- and his arm- against its legendary reputation, he established that he is both fearless and lucky. Sandai Kitetsu could be said to be proof of Zoro's incredible luck. At the same time, it is a blade which Zoro can regognize sight-unseen. He knew the blade was cursed without having to be told its history, and could tell he was holding Kitetsu when his head was stuck in a chimney in Water Seven. I'm not saying that Sandai Kitetsu is Zoro's equivalent of Stormbringer, just that there is a bond between them. I doubt it would leave his service without getting the chance to do him a bad turn at least once.
