Questions & Mysteries Oda's "Hidden Secret" yet to be Found!

[Salah WG]

Hey guys, i created this Thread just to remind Fans of something that Oda once teased but rare few heard about.
There is a Book called "One Piece Doors!", there are so far 03 Volumes of it, and it's basically a Collection of Chapters Cover Pages
However in one of them, it was teased that one day Oda will reveal a "Secret Mystery" hidden in Cover Stories, and till this day it's not Revealed yet

So share your Opinions on this Mystery & tell us if you have any idea what could this "Secret" be
PS: It's a Mystery related to Cover Pages in general, not just a Single Cover Story

 

[Master OF Haki]

I don't give a shit.

 

[Finalbeta]

I hope it's not related to Sanji and Smoothie :p

Have no idea realistically, first thing coming to my mind would be something related to Im sama's palace perhaps.