A nightmare of micromanagement: Taking 5 turns to establish in a new city is a pain, meaning you will spend as much time waiting for them to establish as you will actually benefit from their abilities.
There's a mod which cuts all down to 3. I find it a requirement for any game below Standard speed if you're playing with GS governors.
Remove city states. There's no good reason for having artificially-limited AI powers and all the quests and stuff are just fiddly bits that don't add to the game.
Strong disagree. City States were one of the best additions to Civ and properly matured in Civ 6. They should be improved further, rather than removed. I agree with the now often voiced opinion that city-states, barbarians, tribal villages and free cities should all mesh into a unitary system.
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As for what to improve... we'd all be repeating the same thing eventually if we focused on the most important stuff, so I'll do like most in this thread and find something not yet mentioned: PROMOTIONS!
Good lord are promotions insanely dull in this game... in civ 4 and 5 they were simple and straightforward. There was nothing special about them, but they worked.
In Civ 6 they attempted to go for something unique but somehow ended up with something worse. The trees often do not even make sense. Drill I, Drill II and Drill III makes sense to be set up as a tree, but what logic is there in having
Amphibious as a level 2 promotion and
Zweihander at level 3? It's arbitrary.
I find them so ugly and often counter intuitive. Want your ranged units to be more useful in assaulting enemy cities? You first need to unlock
Garrison, which is a promotion which is very useful when playing defensively.
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So either keep it simple, like Civ4, Civ5, or Oldworld (but not Humankind, preferably. their veterancy system is too basic), or create something new entirely.
E.g. units fighting on hills get better at fighting on hills. Units fighting on deserts, get better at fighting on deserts, units fighting on forests... you get the picture. These would be generic "veterancy" promotions which you'd get passively by interacting with the environment. But then you'd also have more unique ones. Any combat action you take gets you points towards unlocking a specialty promotion. You'd then have a list of Tier 1 specialties, tier 2, and tier 3, independent of the generic combat veterancy bonuses which I mentioned at the beginning of the paragraph. These would include: Swift (+1 movement), amphibious (no embark/disembark penalty), the ability to cross rivers without penalty, ability to build basic improvements, ability to move after attacking, ability to heal after moving or attacking, etc, etc, etc... there's a bunch of stuff one can come up with.
Anything but Civ 6 promotion trees.