You never know where a good idea will come from. People sometimes ask game designers "How did you come up with this?" and the answer is always either unique or unknown. Game inspirations come from a combination of all past life experiences and are triggered by some new one. It could spring from playing a minigame and thinking about ways to expand it, or a theme that looks appealing, or even some memory of playing with toys all those years ago... An idea may even come about from boredom. I know I thought up a few on long car trips. Just leave yourself open and write down anything that comes to you ASAP. You are the only person on Earth with whatever idea you come up with and it's up to you as a developer to choose which of the thousands to bring to life.