Though you're leaving out part of that.
Empire building options, not empire mode. This is far more likely to be new mechanics in the gameplay itself. Some new system that intertwines with conquest or ruling.
There is no "empire mode", because the Pantheon is this game's version of it, and the entire game is part of the Pantheon system. No matter what you play, be it story realms, custom realms, or even multiplayer, it all is part of the Pantheon system. There is no separate mode for this anymore, the entire game is this mode.