When 6 came out, Climate Change was the talk of the town, and they made an expansion about that.
This time around I hope they tackle migration, which is the hot topic these days. Could tie in nicely with trade, open/closed borders, growth mechanics, cultural or loyalty influences, etc. Lots to play with here!
I PRAY that they re-implement vassalization and colonies, those are tremendously important concepts in the history of the world, and lots of fun as an imperial ruler in Civ. As an aside, I also want to be able to grant autonomy to my cities, so that they manage themselves for a boost to happiness, and I only get to give them a priority (military, finance, research, growth, or culture)
I hope canals make a comeback, those are so important to ensure your navy's utility is maximized. Building on that, navigable rivers and no district-based shipbuilding (such that only a city center adjacent to water - navigable rivers included - can build ships). Additionally, I should be able to build transport ships, allowing up to 3 or 4 units to embark and move vastly faster on the seas and oceans, in order to keep up with naval warfare units.
What I'd like to see is the possibility for individual cities to capitulate. When an undefended city is attacked, the troops just march in, which prevents destruction of buildings, loss of population. (Although foreign occupation would play with the migration mechanics and induce exodus)
Internally shared food and minerals: a common theme in history/geopolitics is to describe a given territory as the breadbasket of a region or polity. It should be possible to have cities share food and resources (which would contribute to growth and production) once they are connected, doing so more efficiently when connected by rivers, railroad, modern trucks. The Civ 6 internal trade system is just too clumsy. With such a system, I could have a breadbasket city for my empire, another dedicated to mining and yet another to manufacturing/trade/culture/science (say, where trade routes intersect).
Quasi-wars: I think it would be interesting if you could take offensive action such as attacking shipping lanes and vessels at sea, or combat between land units on a third-continent (not your or your rival's original continent) without declaring an outright war.
Edit: Oh, I wanna add another!
Externally shared science: It's well understood that civilizations that interact with each other IRL develop roughly the same technologies. IME, science-focused Civs are always OP. Thus, though a science-focused Civ in my system may still benefit from being a trailblazer and obtaining a tech before others, once that tech is discovered, civilizations that interact (by trade, war or migration) with the discoverer should get significant bonuses towards that tech.
This introduces some diminishing returns on trading and "training your enemy" (as Napoleon said, "Do not fight too often with one enemy, or you will teach him all your art of war."), as well as on isolation which would now come at the price of limited scientific development. It is understood that relative isolation is what led to China's technological stagnation. Having dominated its region, it was under no pressure to innovate.
I don't think eurekas should make a comeback at all, they force the player down a railroaded storyline. The player's circumstances should guide their scientific development. After all, it is said that "necessity is the mother of invention".
