What games did you enjoy playing in the past yourself? Try to determine why you liked them. What game mechanics did it use that appealed to you?
Assuming that you plan on developing the game on your own or in a small team, make sure you decide on making a game that you yourself enjoy playing. Trust me, it takes a lot of self discipline and motivation to develop a game from begin to end. You'll be playing your own levels hundreds of times before you actually release the game, so you better make sure you enjoy the game yourself.
What works for me is playing some of my favorite games at night, specifically old ones on the Znes-emulator, and then analyzing what I liked, but also what I didn't like about it. Then ponder some more on this before you go to bed. You'd be surprised about the ideas you have when you wake up.