Who says you need completely novel gameplay mechanics to charge money for an app? Surely more polish, hand-built levels and an app are sufficient. Whether OP would make more on ads or IAP remains to be seen, though. An IAP to get more levels would be a good way to hook users.

People charge for Chess apps. :)

Angry Birds wasn't a new concept either.

I was looking for this, I knew I had played it before.

This one has much better controls on the web app, as well.

exch, you point to the mother of all "we stole your game play and made a new game" sites. :)

I saw someone charging for a game based on the exact same principle last week. No reason not to.

This only works if their site gets views and the free demo gets plays. In my experience, apps in the App Store are primarily found through the App Store, meaning potential users won't hit the site first to try the demo.

The standard solution is to give away a bunch of levels for free and charge for additional levels using IAPs.