The thing that makes the 911 great isn't the rear-engine thing, it's the fact that it's a Porsche. But honestly, I've been hearing whisperings of 911 Carrera owners selling their cars and getting a Cayman S instead, because the little mid-engine is really laying it down and kicking some ass. It would seem that, in making a Boxster coupe, Porsche accidentally came up with a car that has more potential than the 911, and that's why Caymans have traditionally been underpowered, and for many years didn't even come with a limited-slip differential (which is shocking, because the LSD was available on Dodge Challengers in the '70s). I think if they were to make a "Porsche Cayman GT2," it'd be one of the best cars ever made.
As for rear-engined dirtbikes, I'd like the audience to consider the implications of making a front-engine dirtbike. Our bikes are mid-engined, of course!