How about this.
Interest on the resources you have in stock. Currently there is only one way to go, if you have it, spend it as fast as possible.
With a moderate interest level it would be helpful to build up a stockpile and then keep levels high, because it would help generate more income. This could lead to exponential growth, but it would take so many hours before things really started getting out of hand. You could also cap the amount you could gain through interest.
This way you could still spend spend spend, but in some situations it would be helpful to build up a bit of a stock pile.