This happens with all Civ installments, it took several years iirc for Civ VI to surpass daily Civ V player counts. A brand new installment is a way for the dev team to try something completely new, building off of what worked, and what fundamentally didn’t, from the previous versions. I would be shocked if Civ VII didn’t include districts (that would make Civ VI look really, really strange in the future), but I’m sure there’s something they want to redo. I personally see them nixing disasters from the base game and modifying governors/loyalty, which took until Gathering Storm to get right.
Things they should keep but expand:
1. Both spreading religion and obtaining tourism for the new win condition for a Culture Victory.
2. Keep District system. Somehow make it to where districts do not look disjointed from city center. Maybe you build districts and wonders inside the city on a completely different map?
3. More Mutually exclusive buildings in districts: Caravansary for more trade routes instead of Market, Opera House for more Writing and Music slots while Broadcast Center focuses on culture/tourism from Rock Bands?, Split Research Lab into Biological, Chemical and Physics, Harbor turns into Naval Base or Cargo Port late game etc.
4. Civics Tree- Although I love a whole separate tree for culture, I think it would be interesting if they went back and did the tree similar to Civ 5. That being said you would still unlock different social policies, governments, wonders, districts, and buildings in these trees with culture. Honestly it might make more sense as a web with a "Code of Laws" in the middle and branch out from there. Realistically not every civilization developed things like truly organized religion, or even advanced forms of writing, art or music, and that would be reflected by the choices you make. It might be too hard to do the same thing with the technologies though.
Things that they should keep for the base game in Civ 7 that weren't in the base game:
1. Monopolies and Corporations- Tie into a new Economic Victory.
2. Barbarian Clans- Make tribal villages the same. Interactions with tribal villages should turn them into city-states too. Would be cool if barbarian tribes/tribal villages had historical names based off of non-playable ones
3. Timeline- One of my favorite things to come out of R&F, even though it had nothing to do with gameplay.
4. Labelled Features- Even if natural disasters, and specifically volcanoes, don't come at first I still want every civilization to be able to name geographical features such as rivers, mountain ranges etc.
New things:
1. Health Mechanic-Probably will take a few years before Civ VII and by then probably the pandemic will be over and it would be crazy not to include some sort of health mechanic to go along, or completely replace, housing.
2. As
@Zaarin said above, I would love for them to have an ethnic mechanic that would tie into culture and might replace, or be similar, to the current loyalty system.
3, Nomadic start and "Prehistoric Era" like
Humankind? 