In chapter 932 Shogun Orochi’s dragon form is clearly shown.
After expulsion from Heaven, Susanoo encounters two “Earthly Deities” near the head of the Hi River, in Izumo Province. They are weeping because they were forced to give the Orochi one of their daughters every year for seven years, and now they must sacrifice their eighth, Kushi-inada-hime.
The story goes that the warrior Susanoo got Yamata no Orochi drunk on sake and killed it, in the process saving Princess Kushi-inada-hime.
He started by chopping off each dragon’s head then to make sure it was dead he cut the rest of the body into pieces. As he chopped off one of the tails, something inside it caused his sword to break. Susanoo groped inside the dragon’s tail and drew out an even more powerful weapon. He took this back to Heaven and turned it over to Amaterasu. The name of the legendary sword is Kusanagi-no-Tsurugi(meaning “Grass-Cutting Sword”) that was originally known as the Ame-no-Murakumo-no-Tsurugi (meaning ‘Sword of the Gathering Clouds of Heaven’).
So now let’s get back to the point. As we know Zoro is going to lose Shusui in Wano as he got it from Ryuma and maybe he will give it to Ryuma’s descendants but if this happens Zoro will need another sword.
As we saw in mythology, Susanoo got a new sword more powerful than the older one, so in the same manner Zoro will get a new sword after killing or defeating Orochi that will be more powerful than Shusui.
Shusui is one of the 21 O Wazamono grade swords.
There are twenty-one Great Grade Swords. Their quality is one step lower than the 12 Saijo O Wazamono, the 12 Supreme Grade Swords which are the highest quality swords.
Only one Supreme Grade Sword was ever shown and it’s the Yoru currently owned by Dracule Mihawk.
To become the greatest swordsman Zoro will need a Supreme Grade Sword and maybe Orochi have one.
Some may say that Zoro will get the Nidai Kitetsu but Nidai Kitetsu isn’t a Supreme Grade Sword.
*Theory by mayank_d_luffy