I think a good idea would be a calendar where you can schedule lessons for yourself, or possibly pick out pre-made lesson plans (beginner, intermediate, expert, folk, metal, etc.) I think this would be a good way to stay motivated and also keep track of what you've accomplished. That way, every day when you brought up the site you could go to your calendar and see what lessons you should do. You also wouldn't have to do those lessons at all if you didn't want to, and could still use the site normally... anyway just an interesting idea.

P.s. If this already exists feel free to pimp-slap me.