Firstly, I think that the 1UPT system should be changed. Ranged units (pre gunpowder?) should no longer be able to defend themselves, but CAN stack with other military units (only one).
I would also introduce a unit threshold. This means that there is a maximum amount of military units you can build. The original threshold is four. Cities can increase this threshold, relative to the city size. A size one city would increase the threshold by one. Four by two. Seven by three. Ten by four. Thirteen by six. 16+ by seven. Improvements can also be build to increase the threshold by two, but they give no yield and must be worked in order to give the unit threshold bonus. Up to two units can stack onto these tiles, and said units will require no upkeep when on said tiles if connected to a trade route. If the threshold is at its cap and then for whatever reason is reduced, combat strength will be dramatically reduced.
The two system above are designed to reduce a massive problem in ciV, and also add more strategic depth into the game. By adding a cap to the amount of units that can be easily controlled, congestion is reduced, but good players can still make a large army if desired. Congestion is also reduced by changing the ways in which 1UPT works, without undermining the underlying principle and tactics behind it. With reduced congestion and large armies logistically possible but hard to maintain, this area of the game should become far more balanced, challenging and fun.
I think it is vital to reintroduce health as a local variable in ciV. Health would work very similarly as it did in 4: buildings, terrain and population reduce health, other buildings and resources increase health. Health is also reduced by city proximity. The closer cities are together, the more unhealthy the cities are. This does not nerf ICS, you can still ICS if you wish, it now just becomes more complicated to control. The current ICS nerfs should therefore be scrapped. ICS should be a viable and powerful strategy, just not so much of an easy one. Healthy cities should grow faster, unhealthy cities should reduce production and gold in a city, proportional to how unhealthy the city is.
A very unhealthy city should also run the risk of diseases. Diseases have a 10% chance of spreading to a new citizen in the same city per turn and a 20% chance of killing a citizen per turn. If a citizen remains alive for three turns, the citizen recovers and becomes immune to said disease. Once a citizen becomes immune to a disease, all new citizens in the city also becomes immune to the disease. Diseases raise unhappiness, which can be kept as a global variable as long as there is a local one. Diseases can also spread to other cities, although this is very unlikely. The likelihood of diseases spreading to another city is based on:
1 - city proximity. Cities closer together run a greater risk of catching diseases
2 - city health. Unhealthier cities also run a greater risk of catching diseases
3 - trade routes. cities connected to a trade route run a far greater risk of catching a disease.
To stop a disease spreading to other cities, a city can be put under quarantine. Cities put under quarantine contribute to global unhappiness. All trade routes to these cities are automatically cancelled, and any resources in that city are no longer distributed. So you really do not want to have unhealthy cities.
Another local variable should be gold. Surplus gold from a city is added to the global variable; negative gold is similarly taken away. A city generating negative gold should not be able to fund its buildings' maintinance.
Buildings should also require workers. Instead of buildings magically running themselves, buildings should require citizens in which to run them. These should be assignable obviously. Buildings with no citizens to not require any maintenance, but obviously no not work. Citizen growth obviously would have to be quickened in order to compensate for this. But this adds more depth to buildings. It really makes you consider whether to build a building: 'am I willing to sacrifice a citizen or citizens that could be working land in order to use this building?'
Like strategic resources, food and luxury resources should also have a limit. Luxury resources should be distributed nationwide and in time should run out. In a 5-city empire, this should take about 120 turns, and of course this is reduced for larger empires and increased for smaller ones. In order for a city to use a resources, it should be connected to said resource by a trade route. This makes road maintenance more worth it, seeing as at the moment they seem too expensive. Food resources should contribute to health. They are renewable resources and therefore never run out, but can only be supplied to a certain number of cities. One source of cow cannot feed an entire empire. Only a few cities. The player chooses where the food is distributed.
Culture expansion should be controllable. Similar to buildings, you should be asked where you would like to expand to next, how many turns it will take, and how much culture it will cost.
Monuments as a new improvement should be able to increase culture by one. When connected to a trade route, they should be able to increase commerce by one and culture by one. This aims to make culture a more important aspect of the game and would allow cities to be culture specialised.
Trading posts, when connected to a trade route, should also instinctively increase commerce (by one). Cottages should also be reintroduced and grow at the same rate of civ 4. There is now a choice between a quick gold boost with a trading post (increases commerce by two or three) or a long term investment with a cottage (begins by increasing by one, ends up increasing by five).
The tech tree should be more clearly distinguished into different routes. There should be cultural, scientific, economical, and militaristic paths in which techs take. These paths slightly interwine: some techs from one path is required for another tech in another one, and many techs in one path increase research speed in another. This allows a civilization to become more specialised in terms of its research, but does not make the tree too streamlined or boring.
Buildings should also operate under similar paths. You can go down different building routes in order to make cities more specialised. Imagine these two system interwined. Now you can really specialise a civilization.
That's all I can think of ATM.
Oh! I forgot one of the most important changes. Diplomatic victory is very dull and easy ATM. In order to win one, you should be nice to other people instead of buy out city states. City states no longer get a vote. The amount of votes a civilization gets for resolutions or for a winner should be decided by their diplomatic influence. Civs earn higher influence by being generally nice and fair throughout the game and lose them by being a d*ck. A long term relationship with civs now means even more. It gives you higher influence. The person who builds the UN is always ellegible for a diplo win, as are the top civs in terms of diplomatic points. Civs not ellegible therefore can not vote for themselves. Just think how much more worthwhile and rewarding a diplo victory would become!
Moderator Action: Merged.