This is my concept for Civilization VII. It could even work as a DLC for Civilization VI, but I think the changes are a bit too drastic for that.
Prehistoric Era
Start with four units: 2 Hunters and 2 Gatherers. These units could create an outpost (similarly to Barbarian outposts in Civ6), where they could produce more units. Both units could lose a percentage of their health in each turn (to symbolise the difficulty of survival without agriculture), which they could earn back by hunting and gathering, respectively. There could be animal units around the map, similarly to barbarians in earlier Civ installments. Some of them could be peaceful, some of them not (they attack player units).
Hunters could kill these animals, which increases the health of all units by a specific amount (could be optional to micromanage or do automatically). If Hunters are able to completely surround an animal unit, they can
domesticate them and place them as tile improvements.
Gatherers could increase health by
collecting fruits from forests. They can
domesticate plants by performing a domestication action (if I remember correctly, similarly to Workers in Civ5), taking some turns.
After
domesticating at least one kind of grain, the ability to train a settler is unlocked in the outpost, and the game transitions into the Ancient Era. (Optionally, it could be the first settlement that determines which civilisation you are playing.)
This concept would also mean that Barbarians leave the game, and each tribe is essentially a potential civ.
Changes to Geography, Climate and Disasters
The current climate and geography model, while not bad, definitely needs some improvement. We need a much larger scale of geographic situations, as currently, the consequences of climate change are limited.
I would recommend having a
global temperature scale, which ranges from “completely covered in ice and snow” via “all ice melted” to “completely dried out”. This would allow for basically limitless climate change, as well as putting civs to test in different environments. This model would also need
biomes to shift with climate change (keeping the Snow - Tundra - Grassland - Plains - Desert categories would be fine). These could shift with a more realistic climate model. (I recommend checking out this channel, it explains how climate could be quite well:
https://www.youtube.com/channel/UCeh-pJYRZTBJDXMNZeWSUVA) Also, it would be great if each civ would have a
“home climate” in which their units and cities are effective, and others in which they would be less effective, maybe by more movement, more combat strength, more production in the home climate and less in foreign climates. (Think of it as the “Russian winter effect”.) If combined with the above suggestion of the Prehistoric Era and the appearance of wild animals, climate change could also involve plant and animal migration, evolution or even extinction. Also, let’s have ice ages as well, not just global warming.
With a wide temperature scale, it would also be important that
all tiles have an altitude value, which determines when they are submerged and when they are not. It is a bit strange that it is impossible to have sea level rise change more than three meters. It literally changed more ca. 12,000 years ago.
The current disaster system is fine (I don't have Apocalypse Mode yet),
meteor strikes are OK if their size and probability distribution matches real-life probabilities. One disaster that is really missing, though, are
earthquakes, which can have dramatic effects on history (see Portugal), much more than meteor strikes or tornadoes. Volcanic eruptions could also have a much larger effect, like some of them causing a
volcanic winter (like in real life in 1816), and thus famine. EDIT: Allow volcanic eruptions to completely wipe cities off the map like meteor strikes (leaving behind some archaeological sites on the map).
Pandemics could also be a disaster which decreases population significantly. The probability of pandemics could increase with the number of animal farms in the country's territory. Trade routes as well as other units could spread the disease to other parts of the world. It could also spread on its own to neighbouring cities.
Of course, nukes are the most visible WMDs, but
biological and chemical weapons could be added as well. EDIT: Nukes should also be able to completely wipe off cities, and not just do some wall damage and a few turns of radiation pollution.
Changes to Loyalty, Free Cities and City States
I recommend
basing loyalty on amenities, language, and proximity (and maybe government form, religion, geographic features, as well as other factors). The most important component of these would be
amenities in the early game, and shift to language in the later game.
Language could work in a way that it
spreads via trade routes, but could also be artificially increased by migration (i.e. sending Settlers into a foreign city would increase its immigrant population). Languages could also develop, split or go extinct, this could also be affected by the kinds media the civ has already researched (books, newspapers, radio, TV, internet) It could even be something that replaces religion, but I think that would mean that we lose out on an important factor of world history.
I would also like the free cities and city-state concepts changed up a bit.
Instead of free cities, we could have new civs emerging from rebellion.
An option would be that
each civ has some “rebel civs” attached to it, which could form by seceding from that civ. (For example, when playing as Britain, revolting cities could turn into the United States, Canada or Australia, or join another civ.) This, however, could probably seen negatively in some markets (let’s say that you are China, and you see Taiwan or Tibet seceding from you).
Another solution could be to
make some fictional leaders maybe going as far as creating constructed languages of just the few sentences they speak. Not that it would be harder than finding someone speaking ancient Egyptian fluently.
City-states make sense a little-bit (given that they usually don’t form as gaining independence from someone). But they (with a few exceptions) weren’t influential enough to change world history drastically. Thus, I think it would be
best to leave them out.