There were some additional in-game features that I forgot to mention with respect to "fleshing out" what is already there.
I love the changes they have made to Strategic Resources, but feel they could go even further in a new Expansion. Like Civ5, maybe have resource deposits of different sizes (from +1 per turn to +5 per turn). By the same token, I feel like there are lots of existing-& new-buildings that could consume strategic resources to either (a) boost an existing building yield (for example, a workshop could consume 3 Niter or 3 Iron & thus boost the Workshop output by +2 hammers per turn, for each resource consumed), (b) generate a whole new yield for that building type and/or (c) create a single new manufactured luxury (like jewelry, toys or luxury clothing). By the same token, I'd like to see them further flesh out Luxury & Bonus resources-by either allowing them to accumulate in the same way as strategic resources (in which case you could have new & existing buildings that could "consume" these resources in return for an extra benefit for that building), or by having new & existing buildings that generate greater output for every 1 source of a resource you have in the city radius (the way Water Mills currently work-like a Meat-works that gives the city +1 food for every source of Sheep, Cows and/or Deer in the city radius). This idea also fits in with what I said above re: new Buildings for the City Center & Neighbourhood Districts.
I also think that the Culture/Tourism side of the game could benefit greatly from some of the best parts of the Brave New World Expansion for Civ5-which I already alluded to with greater Themeing Options for a lot more Cultural Buildings & Wonders-but also with having Tourism/Culture play a greater role in City Loyalty-along with differences in Government/Religion/Ideology, as was the case in BNW.
Oh, & definitely have specialists be a lot more meaningful, in terms of both building outputs & GPP generation.
Oh, & please give us Great Theologians for after all the Great Prophets are used up. Take a look at Rule With Faith to see what I am on about.
A few additional governments to choose throughout the game-especially an option outside of Chiefdom-would be welcome. More types of cards/slots (like a slot for Labour and/or a slot for religion) could be nice too-though not essential.
I think that is pretty much everything I can think of.