I don't like to pass any sort of judgment on ideas before they're completely posted, but I have class in 10 minutes, so let me just throw in my preemptive two cents. The system you describe certainly looks interesting, but I'm worried that it will cut down on uniqueness. If I can add my own little bit to it, I'd say that, in your copper and pottery example, you might instead have a base, more expensive tech ("Warfare"), and this tech would be a necessary requirement for all tier-2 unit techs. Each of these tier-2 unit techs would then have their own requirement aside from that (mining for axemen, hunting for archers and hunters, etc.). Off to class I go.