It depends on what kind of game changer you want to write. If you're aiming to create a game-changing story, you'll need to come up with an original plot that will captivate your audience. Alternatively, if you're looking to design a game-changing piece of gameplay, you'll need to innovate in some way that makes your game stand out from the crowd. Whichever route you decide to take, the important thing is to be creative and think outside the box - if you can do that, you're well on your way to writing a game changer.