Hello,
I’ve discovered this fun type of game, it could be another possible game idea.
As I understand it it’s a 0 player game, you just set it up watch how it plays outs. The game is about marbles in a 2D physics environment and they compete in some way. Simple yet very fun. Obviously the most straightforward idea is to have them race from start to finish, e.g. like this:
But you can come up with many more complicated games, like this “multiply or release” kind of competition:
It’s kind of like pocket sports TV, you just pick a marble and then root for it. Or you may get together with friends and each one picks a marble and then you run the game and see who wins 
Basically a glorified coin toss, but fun to watch.
Implementation would require some tiny physics engine, but as we’re dealing with circles it wouldn’t be anything extremely difficult.