Some thoughts (including things that you guys suggested and I liked):
- An implementation of politic instability/empire cultural diversity (areas with a distinct cultural identity more likely to cause a massive revolt/declare itself an independent state).
- The ability to organize a syncronized action plan with your allies (same goes with the enemies). The "Why dont you attack X" is a really simple form. How about being able to create "unions" with select civs... For example the ability to create something like European Union with some of your allies, where there are programmed meetings and urgent meetings (for example when a war starts). In these meetings the union decides its stance (on a war: setting a commercial embargo on the attacker, sending troops, offering rewards to stop the war etc/in peace time: adopting a certain set of civics, sending economic help to weaker members in the form of gold/free trades-techs). Of course in order to create a union in the first place you need to have a certain status among other civs.
- The 2 time spans/adjustable time rate idea mentioned. 500 years to move your troops into position is a bit unrealistic. Units shouldnt take long to move.
- The implementation of the military as a city distinct slider of hammers, with a top limit defined by your population. Done by adding one more variable on the city output (money,beakers,culture,espionage + military). In other words cities dont build troops in the way they are now. They are a product of them just like beakers are and are stacking inside them (in a treasury style,stacking soldiers). If soldiers>1000 you can divide them in different groups of different sizes (1000,10.000,50.000,100.000...) and name them as you like. Military experts, on top of experience add +hammers to military production. To explain it better:
City X has a base production (hammers) of 10. If we set the city's production military slider to 70% then 7 hammers of the city are directed to produce troops of our choice, which gradually stack inside the city, and 3 go to buildings/wonders.
In addition, to ease micromanaging, an optional military auto-upgrade/managing feature in the military advisor screen (basically a slider determining how much money is spent on upgrading your troops and doing the upgrades accordingly and an interface where you can determine the synthesis of your army) would be needed. It really makes much more sense that way.
To make this whole system more easy to use and different than the current one, add a smart global military hammers slider that prioritizes production rich cities. In other words have production rich cities run a higher military slider and commerce/research cities a lower one, and the sum of all of them would result in the global slider value.
This last feature (city distinct slider/smart global slider) could expand to research,wealth, culture and espionage too but now it would be about commerce, not hammers.
This way we have a more manageable army and more specialized and agile cities.
- Spies gaining experience like normal troops, having their own line of promotions.
- Great Prophets can mass convert cities.
- Mercenaries.
- Cities sharing/transfering a maximum amount of food/hammers (limit depending on the distance from source city + city population. Cities that had food in their BFC and grew up can get food from farther away).
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- Later in game there should be a modifier that accelarates the building of outdated buildings in a city, such as granaries, courthouses, lighthouses etc.
- Auto patrolling (with waypoints, whatever)
- Civics have a heavier impact.
- More world wonders (I know Civ 4 has more than enough but I love them), more often random events.
- Migrations dependent on culture/happiness. Both world wide and inside the empire.
- Radar tile improvements, available with radio and improved with later technologies, providing a big line of sight and early warning of impeding attacks. Give you the ability to move 3 units as if you had 2 turns instead of 1. Great for coastline defence.
- On the modern times, cities gain another gold source, tourism, which depends on its cultural score and the civics run.
- Forts are conquered just like cities are. The enemy has to lay siege on them and when every defender dies, they can take control of them. They provide a big defence bonus (something like +75%). They can also be upgraded later to:
a)Flak towers, which provide great air defences and an additional +20% defence on the units garrisoned inside them
b)Military bases (naval, air and ground), which can produce troops (using hammers from the nearest city), ground bases can also draft some units, air bases can airlift units (+be a base for 6 aircrafts) and naval bases can repair ships faster than normal.
c)Fortifications (like the Mazino line on the French borders), providing +20% against armoured-gunpowder units and do not remove the forest of a tile. They do not produce any culture but are taken in account for the power stat of a civ.
- Planes have an extended radius/power and mission list. Missions like precise strike (90%-100% chance of destroying a specific building inside a city, which could also extend to artillery-missiles-ships) and unit drop are needed imo. Of course that power boost would come with a balanced anti air boost.
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Some other ideas (many not mine)
I dont really like the idea of an ingame battle RTS. If I want to play real time I pick an RTS only game that focuses entirely on that. Civ is turn based and focuses on that. Anavoidably by doing RTS they take time of development from the turn based and thus either the classic turn based gameplay would lack or the RTS feature would lack. After all Firaxis is just another company with limited time and resources on their hands.
Generally the key to a successful Civ 5 would be ease of modding, complex/realistic gameplay (which it already has), a simple/easy to use interface and a large variety of different balanced units/technologies/resources/leaders etc that give the player a lot of strategies to choose from and a lot of depth. Put some nice graphics to that (showing alive cities, and busy roads that reflect trade routes) and you got the perfect game.