This is an in-depth analysis of the possibility that Shiki is Oda’s “Lurking Legend”
Here is my basic list that leads me to this belief, in the order of most-least convincing:
1-Oda intended on Shiki being a strictly canon manga character… or at least that’s what he says. After Wano (which I think we mostly all agree will be the longest arc yet), I imagine that the story is going to speed up quite a bit and I don’t see a more plausible place for Shiki to fit in the story. There was also the cover story of Crocus having a drink with a man who bares incredible resemblance to Shiki.
2-Shiki IS a LURKING LEGEND – Not many characters thus far fit this description, Shiki being Roger’s rival makes him quite the Legend, while being a canon (again, Oda’s words) character that we have yet to see beyond Chapter 0 is certainly lurking. And in his own words, he planned to “lay low for a while”.
3-Floating things – First we have Bege’s ship in chapter 594 in an anonymous area of the New World. This is amidst a section of 594-595 where Oda deliberately shows various Supernovas in the New World, most of whom seem to be in deliberately anonymous but important locations. Each one of these will of course be touched upon in Oda fashion, and it seems likely that Oda has used this scene to foreshadow the important locations of the New World. I have seen convincing theories that the island on which Apoo is running from giant boars and finds himself walking on air is actually Elbaf, and it seems very plausible that Bege was approaching Wano. Bege’s floating ship could only be explained by two possible powers that we know of thus far: either Shiki or Fujitora’s devil fruit abilities. Similarly although less convincing, in Wano we have repeatedly seen a floating rood in the center of Bakura town.
4-Shiki has many similar attributes to Wano – For one, Shiki plans on using animals for his world domination. Second, after escaping Impel Down we know that Shiki escaped to some place where the Government has yet to catch him (Wano is too perfect for this) and is plotting his take over. Lastly, Kinjishi no Shiki has a japanese name and aesthetic. Oda himself said that Shiki has a typical appearance of a Shogun, which I think simply gives him a traditional Japanese appearance.
5-The Lurking Legend is probably related to the Rocks Pirates – This statement is bold and very controversial, but the scene with Garp in the Reverie arc certainly sets up a “lurking legend”, and as I said earlier there are not many of those in the One Piece story thus far. It was revealed that Shiki was part of the Rocks Pirates along with Kaido and Big Mom who have just formed an alliance in Wano. So… could Shiki be this character?
*Theory by Sakkeru