OK, I know I am going to get flamed soooo badly for this suggestion, but I think it could work.
In Hearts of Iron 1 and 2 (which, I confess, is RTS) your people need a certain amount of 'Consumer Goods' to remain happy-with the amount being dependant on the type of government the country had. At the same time, military units require a certain amount of supplies to fight at full effectiveness. Thus, the difficulty for the player is between supplying your units in wartime and keeping your people satisfied. So, here is my idea for CivIV:
1) Shields produced by a city have 3 possible fates-they can be sent to your 'national pool', sent to your production queue for building units and improvements, or left in your city pool as 'consumer goods'. The bigger and wealthier your city is, and the more materialistic your society as a whole is (partly based on government choice, but also based on civic settings) the more 'consumer goods' your people need to remain content.
2) Shields in your 'national pool' have 3 fates: they can either be sent to another one of your cities, traded along with your natural resources to other nations, or used to supply units fighting outside of friendly territory.
3) If you don't have enough shields to supply all your units, or if your units are 'out of supply' (due to being out of range, or unable to trace a supply line back to friendly territory) then their combat effectiveness is reduced.
Please note that I am NOT talking about going back to the system in civ2-where individual cities supported individual units. Instead, like gold support, your nation provides the supply needs of your armies in foreign campaigns. What I hope this idea would do is twofold-first would be to more strongly link war and economics-forcing warmongers to consider their domestic economy when they engage in long, drawn out campaigns. Second, I would hope the idea would link government and civics choices more directly to your ability to wage wars AND maintain a strong economy.
Anyway, hope that makes sense.
Yours,
Aussie_Lurker.