Finding a good idea for a game is not a simple task. Ideas you can search completely everywhere! Almost everything that surrounds us can be something around you will spin up the game. One of the most odjechanych examples I've ever landed is the bread simulator.
Just look at some item, event, whatever and think about how to resist this game. From time to time I do precisely such exercises of imagination/creativity, consisting of choosing something (object, person, shape of ideas) and designing the game based on it.
It's also a good idea to have something on hand that will keep you fleeting. A sheet of paper, a voice recorder, whatever. I always keep the paper and pencil next to the bed, because I'm often amazed at the ideas.
Game designer should be interested in various things, browse the unmeasured resources of the Internet, watch movies, read books, play games, travel, explore, ideas lurk everywhere, you have to see them only!
You can't be afraid to inspire another game. Look at the gaming market. It is filled with games that are very similar to each other. However, it often differs from some element, the scheme of activity. Often, the designer works by thinking about what poprawiłbyś in an existing game.