Post Made Community Wiki by Tetrad
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One informal way is to observe the world around you. Watch the kind of objects and interactions you have every day. See what makes people happy or what captivates them. Is there any way that can be used as the basis of a game? Or can you add it to an existing game in a way that makes something new? Mull it over, try playing a mini prototype in your head. If it doesn't have potential, move on. If it does, make a real prototype.

Spend a lot of time thinking and observing. Eventually you will have a lot of ideas that aren't good enough and some that are.