Unrelated to topic, but anyhow.
Vex has a billion interesting models. Or at least, umake does. Take for instance the commodities system. When modelling commodities, I thought "what basic categories shall there be?"
It turned out four: food, material, fuel, and minerals
This isn't some kind of divine list or anything, just seemed a reasonable enough set.
Immediately it strikes you that it would be totally inappropriate for some cavemen to traffic in deuterium 3 nitrate or some other exotic thing, so we go ahead and parameterize each commodity along cultural evolution as well.
Then, it comes time to place commodities. What I realized just now while puzzling about Dr. Mario on my bed, is there are really two other broad categories: cultural commodities (food and material) which are only trafficked by a certain culture, but can always be trafficked by them no matter where they live, and planetary ones (fuel and minerals). So a planet will have some number of fuel/mineral commodities on it, but only if there happens to be a culture of appropriately high evolution type can it be mined. On the other hand, anywhere there is a city of a given culture, you may be able to purchase their basic food stuff, or construction materials.
The model isn't really what's so interesting, it's how the model is arrived at through this intuitive-organic design process.