@Bando & Camkiaze
I started to dislike many of the CIV features I got introduced to EU4. Incidentally, this has nothing to do with EU4 by itself. I don't want CIV to be an EU4 clone at all. What EU4 proved, however (and please don't nitpick stuff like fort maintenance or naval attrition) is that CIV should be designed in a way that it can be well played by an AI. I know this is an old example, but take HOMM3 combat AI for example. It took some time to actually top the AI in decision making, because the AI knew all the rules and applied them. Was the combat boring for a human player? No.
Civ5+ feature too many game mechanics that the AI can't cope with. Not really. And their answer to this solution was either artificially restraining the human player (which is terrible) or revamping the mechanic completely (which is good).
Combat and unit movement
If the AI can't deal with hex-based 1UPT combat, then ditch the whole concept. Why not? Re-introduce armies. The AI can handle moving armies around. There are various alternatives out there, some almost as old as me. Don't take these examples literally, please.
Centurion:
Heroes of Might and Magic 3:
Battle Isle:
The combat is so slow currently that even a separate "combat minigame" would make the game run faster, even in multiplayer. And air support could be easily added as a feature like "if within X tiles of an airbase with X airplanes, add them to the minigame".
Map size
The map is too small. The number of hexes should be like 4 times as much, or perhaps even more. Again, not a problem if AI moves only armies across the map. It would also allow for "passive units" like fortifications or minefields to be added to the map. It would also allow unique terrain features like mountain passes, canals, rivers that can be navigated by ships etc.
Number of civs
I watched the new BE expansion diplomacy. And I really like what they did there. But the main issue with diplomacy is that there are simply too few civs on the map to make it relevant, especially if you play on a 6-8 player map (small/normal). Have a civ game with 20-30 civs on a normal sized maps and introduction of "pact victories" would make it oh so more challenging and fun. And so we make a full circle to...
The builder segment
I greatly actually disagree with the concept of "terrain features" like citrus or iron. As already proven by Civ6 itself, this is a concept that eventually has to go. Land should have features like "fertile soil" or "wetplains" or "granite" or "pine forest" like in SMAC. And if this is the case, you could also ditch the whole concept of "this building adds +1 to food". What if instead workers could plant Wheat on fertile soil, while the granary just "unlocks" the limit on Wheat fields? This way you could have whole civs specializing in certain goods, resources, trading them away, replacing them with other resource types as the ages go by. What if by the late industrial revolution you replace Wheat on Fertile soil with Soybeans or Oil Fields? Why Not? in That case a Refinery building would not add +20% to production but convert X crude into gasoline.
This is, again, nothing new. Sim City 4 was a terrible game, but had its moments. One of these was the terrain resources and the concept of this economy. Why not steal from SC4? This specific idea was awesome.
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My biggest gripe with Civ5+ is that it isn't really a civ game. Because there is no change. Everything is basically just upgrading what you already built. in CIV4 at least the terrain looked completely different. You could literally see the change of ages. Industrial revolution was railroads, mines everywhere (or windmills!), less farms, more cottages. That transformation was even more drastic in SMAC. Forests or fungus everywhere, or specialized cities, vast fields of solar collector, heavy borehole cities. It was so exciting to change the landscape your civilization (type) thrives in.