Also while your at it, cap the number of cities to 5. Might as well cap the number of soldiers to 50,000, number of tanks to 500 and so on!
Seriously, the worst idea I've ever heard, it just limits the potential of the game in my mind!
I can't see this working based on the fact that the purpose of a police force is to serve the public by protecting property,enforce laws and limit civil disorder. That is to protect ALL properties, Enforce ALL laws and limit ALL civil disorder not those who are able to pay for the service. It just makes no sense in my mind.
Also against this. The system is just asking to be abused. If alliances really want to pursue this, they could simply implement it outside of the game.
Fighters would have a massive boost when defending against airstrikes and directly targeting aircrafts. While bombers would have a massive boost in destroying infrastructure, tanks and ships. Could be an useful addition in my opinion, and of course it would open the door for specialized units of every unit type - anti-tank infantry, heavy infantry etc. The possibilities are endless, but it would result in a very complicated battle formula I would imagine.