Earlier this year, Callum Newman (who goes by "Prince"), Great-O-Khan and Jake Lee made references to there being a hidden "king" behind the United Empire. In his interview before The New Beginning in Osaka, Tsuji referenced the "king" too, and said that if it's who he thinks it is, then they have unfinished business.
My guess is that it's Kazuchika Okada, but I'd like to see what anyone else has come up with!
by ichigosenpai_