My opinion is that the game is its mechanism. Any discussion around this point is just an exercise in theory. When you have a working mechanism you have a game, and when this mechanism is enjoyable, you probably have a good game.
I believe that starting a game from the theme is impossible and untrue. When you say, “I’ll make a game based on Star Wars,” you’ve said absolutely nothing. All you’re doing is procrastinating the process of thinking about the mechanism.