Yeah I kinda realized afterwards, trade + agriculture only sounds like great rules, but I didn't really have time/internet access afterwards .You know that would actually be sort of crazy combine that with the mega city idea. I would have to say building would be hard I don't think any of the villagers sells builing blocks so might need to augment a set of rules for what you are allowed to work with like maybe only allow farmable resources and tradable resources. So tree farm and cobble gen would allow some building stuff.
You know, the three rules would make for a great RP server with economy.