More uses for resources other than gold
Just about everything needs gold. Unit upkeep is also solely in gold. I only focus on gold improvements in my cities because it's the major limiting factor in what you can do. By midgame i usually have hundreds of extra material and more metal than I could use because the needed gold severely outweighs the other resource needs. The AI tends to have thousands of materials too, so it becomes useless for trading. Equiping champions takes a lot of gold and nothing else. Making units reduces your possible gold income even further, causing other resources to be even more worthless.
I suggest adding material, metal, and crystal upkeep on units and reduce the gold upkeep slightly. Also add material, metal, and crystals into the price of champion gear. Instead of thinking of it as shops selling you equipment, they 'craft' it with your resources