Grain Supplies:

I'd like to see some kind of scripting that makes control of a major grain producing region a prereq or trigger for advanced growth and/or high income.
What I mean is The Athenian Empire of the 5th century BCE was predicated on control of trade with the Black Sea and its grain supplies, and Roman expansion came to rely on controlling Egypt and Sicily as major grain producing regions. Without steady supplies of grain neither Rome nor Athens could have grown to anything like the size they did. Could there be a limit to city expansion based on the availability of food supplies? Technology developement follows on: it takes a big city to build advanced MIC and train advanced troops. Without control of the necessary resources it can't be done.