It occured to me that you could probably do this without restructoring the game entirely. Make " Wealth " a tradeable resouce. So - when you have a city producing " Wealth " - " Wealth " appears in your Resouce list on the diplomacy screen. You get one Wealth unit to trade for every city you have doing it.
So a Civ that you have to pay tribute to nixes one of your city's production. And you can nix one of theirs if you're getting tribute.
It's an idea.