Help with a building mechanic.

It looks like you are in an early development stage, where you haven't yet chosen the core mechanics that will drive your game forward.

I would recommend that you get a notebook, and start drawing up pro's and con's for the various mechanisms you can imagine, as you see them. Work it out in your imagination first. Look at primary interactions and requirements, and then push it to a second stage as well.. how would doing it this way interact with a later part of the game...

Take a lot of notes and play around with your ideas. It takes time and nobody can give you "the answer" because your game is still wide open to possibilities. You dont want them to, either, because it is often in this part of the process where you will come up with a lot of your smaller ideas that will grow to make your game special.

When you start to gravitate towards a particular mechanic, then work out the basic rules governing the interactions, looking for holes or game-breakers. If you can build it conceptually, at this point you can then make a physical prototype and start ironing out kinks and _really_ look for the hidden problems that might break the game.

Finally, while it is good to have boundary conditions in mind (30 to 60 mins completion time), and this limitation might help to exclude certain mechanisms, it doesnt necessarily have to control your design strongly at this point in time. Keep in mind that a lot of the operational parameters can be fine tuned at later stages... want the game to go faster? simply multiply the resources that drop, so players gain 3x instead of x. Want to make it more challenging? Change the probability distributions for bad things, so they happen more often. Or make them hit harder. Etc..