I have an idea that could serve you well. You could make units called "mining location"(which would be placed where the gold mines are now) where you can build gold mines like the undead, place them on top!
That starting cart would have the ability to do so, or you can make a unit capable of doing so!
There would also be invisible locations that aint shown at the beggining,
but can be detected by some mine finders, or even upgraded peasants!
Well its only an idea, but i think it would make the game even better!
Tell me if im wrong...
P.S.: sorry for my spelling mistakes