Yo, not sure if this was suggested before or not, I'm pretty new here.
So here's my idea. Why not have a resource based income (raw materials such as iron.) The rule would be that you have to build mechanic units (tanks, transports, planes etc) using raw materials which you get a certain amount in the beggining of the game. And as you play you can mine mountains or whatever. But still have the usual cash coming in to recruit soldiers. And why not put that cash into upgrading your units stats? Better armor, mobility, etc?
Personally thinking, it would be a great idea except that it would be probably hard to manage.
So yeah, this is kind of vague. sorry.