A Wiki entry described the design of Shiki as a typical shogun design.
It also stated that Shiki is commanding a bestial crew similar to Kaido and in addition to that Shiki´s name is Shiki the Lion, which is also a reference to Kaido´s Crew full of beasts.
Then I remembered a cover page (631). The man on that page looked similar to Shiki, the man also wore a hat typical for Wano.
Then I added 1 and 1 together and came up with the idea that Shiki is the Shogun from Wano, who killed Oden together with Kaido.
*Theory by PoorMansRose