I have an idea about pretty highly moddable platform shooter game using Node-Webkit. I think I can solve most things, but I'm not sure how users could set their own sounds and music. There is already the option to play them by name, but I dont think there is any way at the moment to load them at runtime.
Another thing that would be cool if users could use custom images for the patricle effects. Maybe load image from URL or use Sprite image for example.
I was thinking using Spriter for the animations, but I don't think it is possible in a long time to load them externally. So I think I could probably make my own animating software integrated to the game.