Naval/Navy mechanics
I've long had the idea that it would be interesting to see ship-related mechanics implemented in the game. Not necessarily full-blown naval battles—that would probably be too much. For example, imagine a new map type where most of the map is ocean/sea, and on this part of the map, the player can deploy a ship—an expensive unit that essentially serves as a type of artillery, but from the water, not from land. Other ships could be landing ships, for delivering soldiers to land, assuming that soldiers can only reach land on the map via air or sea landings. Scout ships or boats.
Overall, this is just a concept that needs to be thought through, but I think it would be interesting, especially in connection with the new Cuba expansion or the potential Greece expansion. This opens up opportunities for island-based maps with water, where ships themselves play a decisive role.