I like this! Definitely going to be using this in some capacity, so thanks!
... it should be really really simple but I seem to be unable to come up with simple game ideas.
One approach would be to pick a core mechanic and consider it in a vacuum. As to what a core mechanic could be, I like to define it as the game's bedrock; the smallest, most basic unit of the mechanical system(s): say, movement
or jumping in Mario games are each a core mechanic.
Let's say we picked jumping: I think 'Downwell' does an excellent job of taking that mechanic and working with all of it's various states--
before the jump, during the jump, after the jump. I've actually tried playing the game with just the jump button to jump/shoot (with minimal to zero lateral movement). And you know, it works rather nicely. Although the overall system isn't complete without lateral movement, the jump mechanic on its own feels complete.
There's a kind of simple elegance one can get from considering a single mechanic, then wringing it out dry.