What’s the cure for overscope? Go to the other extreme. Design a game that you can do in one week or less. If the game comes out looking good, you can always spend the next week adding another small set of features. If it comes out horrible, you’re not so attached that you can’t abandon it and try again with a brand new project. This does mean an adjustment in expectations – you might not make the next Final Fantasy game, but you can make the next Tetris, the next Everyday Shooter, the next Spelunky!, the next Narbacular Drop.