This is a very good question that plenty of budding game developers hit. We actually recently wrote a teaching document that covers this issue here:
It's a bit long, but your problem is quite complex. Quite simply, you've created a nice little game engine with some features. Which don't get me wrong, is freaking awesome. So many people never get this far at all (Maybe 15-20%?). I commend your efforts.
The problem is you're not really experienced in making an entire game, you've definitely gotta learn this stuff but when professional developers make their games they don't code anything at first. They go through a long pre-production process where they design, playtest, and refine the game. Because every bug you fix BEFORE you have to code, is a whole hell of a lot less work.
I'd suggest you read the thing linked, it addresses the very beginning of a game project and how we went about it for our current little project. Specifically, you probably want to design out and solidify your vision for what you want your game to be, what you want it to accomplish, and let that lead into all the things you have to do to make that happen.
If you got a moment, check out our project on Kickstarter, it's specifically designed with folk like you in mind (We're trying to teach people game development!)