Zoro is currently in posession of three legendary katanas (ignoring the fact that they were temporarily confiscated). Wadou Ichimonji, Sandai Kitetsu, and Shuusui. I'm curious what people think about the katanas themselves.
Sandai Kitetsu:
What we know: Third generation cursed demon blade, one of the 50 legendary swords. The first generation (Shodai Kitetsu) is part of the 12 legendary swords, and the second generation (Nidai Kitetsu) is part of the 21 legendary swords. We know that Zoro can sense its presense, and that it has a thirst to attack against Zoro's will.
Things to suspect or question: Zoro's technique Asura could somehow be directly linked with the katana itself. It's likely that Zoro will encounter the first and second generation versions along the way. Some people believe the first generation katana may be carried by one of the five elder stars as shown here http://www.onemanga.com/One_Piece/233/16-17/ because of the katana's hilt. Would Zoro abandon Sandai Kitetsu to wield Nidai Kitetsu? If I had to predict, one of them will somehow absorb the powers of the other, and Zoro will wind up wielding that.
Shuusui:
What we know: The black blade Shuusui is one of the 21 legendary swords, formerly carried by a legendary samurai who killed a dragon. Zoro has commented on the blade's legendary hardness, saying the weight of a dinosaur would be unable to bend it. Zoro has also commented that the katana is quite heavy and extremely powerful. Lastly, the katana absorbed the power of his other two katanas when he first used his 108 attack with it.
Things to suspect or question: Zoro is worried that Shuusui will be a "problem child" like Sandai Kitetsu. This implies that Shuusui has a strong will of its own. Does that mean that Shuusui is also cursed, or does it simply mean that it's difficult to control? If it wants to display its own will, is there something that it desires?
Wadou Ichimoji:
What we know: The Wadou Ichimonji is one of the 21 legendary swords, formally owned by Zoro's childhood rival, Kuina. The katana holds Zoro's promise to become the greatest swordsman, so its value to him is similar to how much Luffy values his hat. This is clearly Zoro's most important weapon, and it should continue to be throughout the entirety of One Piece. Lastly, it's the only sword that he uses for his steel cutting technique.
Things to suspect or question: Everything. This katana is in the same class as Shuusui, one class above Sandai Kitetsu. Even though Zoro's been using the katana since day 1, we have no knowledge of its abilities. Is his ability to hear the breath of steel somehow related to this katana? Why did Kuina possess such a legendary katana, and where did it come from before she had it? The katana itself seems weaker than its rank these days. Does the katana possess some latent ability that Zoro will come to realize? If so, when? It's possible that Zoro could learn to use haki through this katana.
Just some food for thought, I have a hunch that this stuff will become imporant to Zoro's growth.
