Let’s get logic out of the way first: Shogun Orochi is likely going to be a brand new character that we have never seen before.
But.. What if Orochi is someone we have seen in the past?
To answer that, let’s try and collect all the facts that we know about this elusive Shogun.
1-He showed up in Wano 20 years ago, to overthrow the Kozuki clan alongside Kaido
2-He uses a two sword style
3-He probably has enough strength to work alongside Kaido (this is speculative, but barring a case of Buggy 2.0, it’s safe to assume that this shogun has a lot of personal strength too)
Not a lot to go off on, really. But we can draw some inferences. Here’s what I propose: The Shogun is Shiki, the Golden Lion
Points in favor of Shiki being the Shogun:
1-Shiki was captured by the Marines 22 years ago , shortly after Rogers’ death. He escaped Impel Down and has been at large for the past 20 years.
2-Shiki is shown having considerable skill with his two swords…incidentally, both swords are named swords : Oto and Kogarashi (incidentally, Kogarashi refers to a the first winter’s wind, a word/description that is unique to Japan, something that Wano is heavily based on)
3-Shiki’s desire was to make Pirates the rulers of the sea, and to unleash a plague of mutated animals, declaring war with the whole world. (While it can be argued that this was a movie-only motivation…I think that Kaido’s Beast Pirates are close enough to the mutated animals plot that Shiki wanted to do)
Speculative points in favor of Shiki being the Shogun
1-The floating dome-like structure over the sumo ring. It was certainly odd, and it was floating by undisclosed means. It can be something else, of course…but it’s just as likely something floating under the power of Fuwa Fuwa Fruit. Shiki’s Devil Fruit.
2-We know that Oda wanted to introduce Shiki earlier. As early as Shanks-Whitebeard talk…meaning he was significant enough, or relevant enough to the story. Oda refrained from doing so because he felt it might be too much information.
3-Shiki’s design is reminiscent of a typical Shogun.
4-Shiki’s epithet : The Golden Lion. With Lion being the King of the Beasts, it’s very thematic for Shiki to be lording over the Beasts of Kaido’s pirate crew.
5-The Lurking Legend. Since Shiki has lain low ever since he escaped, 20 years ago, he very well might qualify for the “Lurking Legend” that the StrawHats face. He did respect Roger enough, despite Roger being his bitter enemy, that he stormed Marineford after Roger’s capture. It’s not too much of a stretch that Shiki had some sort of relationship with Whitebeard too.
6-Considering that Shiki’s backstory is canon, it’s a bit bizarre that such a legendary figure hasn’t gotten name dropped more often during the course of the story. Him being present in a closed off isolationist country like Wano, might be a valid explanation to justify that.
Keep in mind, that this is all pure speculation and guesswork. What do you think?
*Theory by WarrenDSherman2