IDK...perhaps a time limit to implement for the patent to be registered?
You can hold a Design Patent for 2 years, after which if you haven't achieved a practical prototype, you lose patent rights.
Maybe something along the lines of a "Provisional Patent Application" (which lasts for 1 year)?
The Other Tesla David Blaza5 comments I find myself going to Kickstarter and Indiegogo on a regular basis these days because they have become real innovation marketplaces. As far as I'm concerned, this is where a lot of cool ...