Love the enthusiasm.
Just remember, the demo (and game) won't be out for quite a while, and we haven't publicly announced how we're going to add mod support.
I'd recommend that if you really want to start making a mod for the game just now, work on your assets. Make them to the best quality that you can, and once the game (+mod support) is released, you can work on integrating those assets into the game.
Best of luck!