Yep, I reckon that a new Ideology/Ethnicity System & Economic Victory mechanic would be sufficient to justify a 3rd expansion (just as Disasters & Diplomatic Victory justified Gathering Storm

). However, just like with the expansions we had before, I feel that a 3rd expansion can be used to enhance existing game mechanics-for example:
-Making the Religion Game more robust throughout the game (for example, by tying it more to Diplomacy, City-State Relations & Social Policy Effects)
-Making Resources more important to the Empire-wide Economy (& bringing back the concept of "Food Diversity" from CIV).
-Make Ages more dynamic (by having Net Housing & Amenities impact the thresholds-along with positive & negative events during the current era), having more Dedications across the game & having Dark Ages be more impacting-both for good & ill-by giving them their own "Unique" Dedications.
-Make City-State Relations more dynamic by having Protection Pledges, Bullying, War Declarations, Religion, Ideology, Corporations & 2nd/3rd Tier Spy Missions impact more on Envoy numbers (both up & down). Also perhaps look at ways to incorporate City-States more into the Diplomatic Victory (without repeating the mistakes of Civ5).
-Expand on the Pollution & Disasters systems introduced in Gathering Storm (things like Over-industrialisation & over-use of mines & wells negatively impacting on Tile Appeal, Tile Yields, Amenities, Housing & potentially even loyalty).
-Bring in a full fledged, semi-random event system.
-Make Loyalty more dynamic, & expand the "Loyalty" mechanic to apply individual pre-owned tiles too (as was the case in CIV), to allow players to steal, buy or trade pre-owned tiles.
-Further flesh out the Great People, especially Cultural Great People, to make them more meaningful earlier in the game.
I'm sure there are plenty others, but I think that list would more than justify the price of a full expansion!
