Two big compliments to Civ IV: city maintenance fees and corporations are not only a great start, they work wonderfully already in many ways. But they do need to be expanded upon, particularly the corporations. And heres why:
[1] Not enough of them, plain and simple. With only two food companies (Sid Sushi and Cereal Mills), theres barely any sense of rigorous competition. Also if theres going to be more of them, they need to cover more different individual resources. There need to be companies specializing in spices and silk, and other (perhaps smaller) groups of resources like that. There are no Oil Companies for example! This is a massive part of modern history!
[2] They need to be more easily acquirable. The idea with Great People founding corporations is interesting, but then more corporations need to be available to more Great People. Great Prophets and Spies cant found anything, for example, and maybe they should. Great Engineers can only found hard-mineral companies like Mining Inc and Creative Con, and maybe that should be expanded or diversified.
[3] They should introduce new resources. Not too many of them, just a few. Tobacco was and is a big part of history, so is Cotton, and so is Rubber.
[4] They should introduce the concept of the manufactured product. They should allow for corporations that utilize natural resources to make finished products. For example, lets say you discover Combustion and have access to Oil, Iron, and Rubber, you should be able to build an Automobile Company and create maybe about 3 or 4 tradable Model Ts, just like Hollywood gives you a few Hit Movies, except that it uses a few existing resources to make better resources.