I dont really want to sound wise by stating the obvious, but it really helps if you come up with some basic goals for a new game before you start developing it. Nothing really fancy or complicated, because you’re bound to change some or maybe several things in the design stage anyway. Just what you want the game to do in general terms and maybe some pitfalls you should to avoid.
So, for the game I’m working on my project statement would be something like “emulate the senseless violence on a massive scale found in a modern FPS video game”. Straight away I can figure out some of the goals my game will have to achieve.
If I want to do big battles with tens of enemies, I have to come up with a simple and quick system that allows a steady flow of play without burdening the game with a lot of unnecessary rolls and calculations. Each character should have a small number of traits controlling his actions to keep things manageable. Combat will probably have to be abstract instead of using a battlemap and miniatures as even the most dedicated and hardcore GM will flinch at the idea of having to move a few dozens of miniatures each turn. But, I will still need to fit big guns and ridiculous amounts of damage and make them somehow work with the abstract terms of the battlefield. I will also have to figure out how to handle things like ammunition and carrying stuff without having to keep track of exact numbers.
All I have to do now is work towards those goals!