E3 is near, sounds like it's something they will announce there. I'm pretty sure it's not an expansion though, so soon after Gathering Storm. Some kind of spin off announcement at E3 sounds the most realistic. That's the last thing i would hope for though, i would prefer if they just focused on CIV VI. The most realistic alternative would be something big in the patch notes like finally better AI or new DLC civs, although i think they won't do that anymore because players didn't like that kind of DLC.