I'm going to try something probably way more difficult than I ought to - a multiplayer game. I've been interested in the io games lately - multiplayer arena games where people join and leave at anytime. There are so many great emergent moments that arise when playing with complete strangers.

They often center around collecting a shared resource in a common area and fighting in various ways over that resource. The best ones have a push/pull relationship with progress. The larger you get, the bigger target you are - so you get a good game experience right out of the gate and the difficulty ramps up naturally as you progress.

It took me a while to think how this could work with the theme of 'running out of power' - running out of power implies to me some kind of defensive mechanic, and I was trying to imagine somehow hoarding your energy and protecting it with traps, but couldn't think of a way to make it relatively simple to grok through playing. What benefit would the player have to hoard? I thought maybe you have a limit to how much you could hold, but then what happens when you progress higher? Does your limit increase?

The key insight I had to simplify it is to use the 'power' you collect to convert into another resource, which you can use to build new things. Having to put the power into a machine that mines another resource gives you incentive to leave your 'cave' to collect power, return to charge the machines, and build traps to protect them.

I have no idea if this will work, but it'll be fun to give it a shot.