Yeah! I think the main two elements that I would love to have a version of in your game is the "main objective" to reach certain "special resource" and have to extract it completely in order to progress or win.. which is going to take you time and resources. And while you are doing so, prepare defenses in a way you can survive multiple waves of enemy attacks. These should be harder every time, in order to make you optimize and recover the "special resource" as fast as you can.
So basically a main objective and time pressure!