The concept is really good! And lots of things can be explored from here.

1. Bring all the cheat sheets, Best Practices and Gamify them.

2. Bring Linux command line Drills, Best practices and Command line shortcuts(I will pay $$ for this!)

3. I miss typing tests and drills. Something better than typeracer competitions?

4. Show the product right away in your landing page, and not asking for registrations.

5. Finally build a platform and Crowdsourced community around. Let others decide what Practice they want and also challenge others.

Thanks, I will be your customer and definitely pay for such a service!