Supercar-wise, I'd also opt for a review of the Mercedes AMG SLS or the Ferrari California. Being a bit of a Saab fan, I'd also like to see the Turbo-X and the new 9-5.
However, what I'd find even more fascinating would be an alteration of the cheap car challenge and the epic journey format, although I'm not quite sure if the following idea would even work. On the one hand, we've had alot of those challenges in which the guys have to buy really cheap cars and then - on the other hand - we've also had those epic supercar journeys like their search for the best driving road in Europe. What I'd find interesting now would be one of those epic journeys with cars that are neither modern supercars nor actually cheap, but in a realistic price range for many buyers. For - let's say - 20.000 to 30.000 pounds, you could buy everything from a classic sportscar like the Porsche 964 to a powerful modern day sedan like a used current BMW 330 or thelike. The reason I'm not quite sure about this idea myself is that it could be that with cars that aren't a heap of junk the challenges wouldn't be funny and without them being real supercars they wouldn't be that epic either, so it might be a difficult concept. It is just that too me those epic journeys some how always captured the sense of driving in a way no other feature did, which is why those are the ones that often inspire me to get in my car and drive somewhere just for the journey's sake. Last time I've checked the garage though, I didn't own a supercar, which is why I always wonder what those Top Gear journeys would be like with cars that I'd actually consider buying and could own for real.
Another interesting variation of this format would be a journey of that kind with supercar youngtimers like a Diablo, a Testarossa and thelike.