We all know that Shiki is indeed a canon character and he’s alive. Shiki was considered the 2nd strongest pirate next only to Gol D. Roger prior to Edd War.
In Marineford War, Sengoku mentioned Shiki to be the first escapee:
As per Shiki:
So Shiki’s plan already started since then before the timeskip?
So it’s confirmed even by Oda that Shiki was already active before the timeskip.
Oda originally wanted to make Shanks and Whitebeard to talk about Shiki which is currently active in doing some stuff. Also as explained, Shiki overall designs represent a Shogun. Oda doesn’t want to continue his original plan because people might confuse Kaido’s power (1 of the Yonko) and the difference between Kaido and Shiki.
There is also foreshadowing in some manga cover:
Shiki has high regards to Roger infact following his death he declares and recognizes him as the real King of the Pirates. He even hates it when Roger dies for the likes of Marines. So I guess it’s no problem if it’s Shiki in the cover page drinking with Crocus.
I even doubt that it’s Scopper Gaban, like many speculate. In short I think Shiki is the Shogun of Wano.
*Theory by Nakama