There is a gearbox out there that has the hop up built into the the gearbox. I can't remember the name of it, but I did have one, and am kicking myself now for selling it.
But yes it is a great system, but if you have ever used a true classic, then you will know that this is really nothing new. All they really did was bring back something that we already knew. But what I think they really did was open people back up to something that many of them didn't know about, or thought that they would never be able to use. Because not many people are able to run a classic rifle anymore, though with the P* you are kind of getting there. While not a classic it runs on basically the same principle, just with newer technology.
I have had the chance to run with a Sun Projects M16 a few years ago at a large game. And man was it a blast!
But I also think that the next "big thing" is going to be a system much like the P* but like was said before, is submersible, and has a functioning charging handle. Then I think you would really have something on your hands.
Make it very reliable
Use easily accessible mags
Very user friendly
Function externals, dust cover, forward assist, charging handle ( when talking about an M4 )
And able to take very common external parts
Make it at a reasonable price to a consumer
Something like that...... I think you would have something great on your hands!