I actually am betting this is already planned but if its not:
It would be nice to have dynamic values for revenue from countries, not the static values based solely on if they are happy or sad at the moment.
Example
America's attitude is exuberant: In month one they give you 56,000 credits.. Month two 57.600 Credits, Month three 58300 credits.
Now lets say they are unhappy.... Month one they give you 40,000 month two 35,000 Month three 31,4000..
Perhaps also factor in UFO sales, as it seems reasonable if a country gets cool toys from you, they would give you more, like add 500 credits a month on to whatever country you sell a ufo to...
This kinda simulates the economic effects of being successful and unsuccessful at defending the planet as well..