I'd like to see a whole review on the CTS-V line up, coupe, sedan and wagon. This will be interesting if we can compare it to a similar car. Would be a fun race too, best way to transport a package.
Rally car challenge - X amount of money to buy and build a race ready rally car. This would be interesting since we have Subaru, Mitsubishi, Audi, etc. I'm pretty sure Ken Block or Dave Mirra would be of some help if the the hosts can ask for it.
American vs Japanese vs German Luxury car comparison - who makes the best Luxury car for US consumers. This could be a new car comparison or a challenge (which host can design and build a better luxury car. This would be similar to Jeremy Clarkson's Merc build)
$25,000-$30,000 comparison, GTi, WRX, Mazdaspeed 3, Lancer, Civic Si, Mini, Focus, etc etc.
Ultimate Aero SSC vs Veyron SS