In my mind, these Zeta cars would be the "thesis" or the "definition" for the divisions these cars represent.
I believe it would be great to see:
-MY 2010 Bel Air (replace Impala after phase-out):
This will be THE Chevrolet. Boldly styled, beatifully executed. Detailed, plush interior. Although this would be slightly more expensive than the Impala, consumers would find an increased value: better packaging, transmissions, power, sportier handling, and better material quality. Ideally, this would be the perfect product that Chevy has failed to deliver for so long...
-MY 2009 Pontiac G8 (replace Grand Prix)
Again, this will be the definitive Pontiac. Although this will take place of the Grand Prix, it will be geared to a more affluent consumer. Ideally, this would offer the ride, handling, and build quality of a BMW 5-series for a fraction of the price. Additionally, these value-conscious buyers will find better acceleration, increased passenger and cargo room, and a LOT better styling :P
-MY 2009 Buick Velite (addition to lineup of LaCrosse, Lucerne, and Centieme)
A four seat hardtop convertible with signature Buick styling (preferably more original than the Velite concept as well as more engaging). Unlike most four seat convertibles, this will offer back seat room aplenty. Primary competitor: Lexus SC, but priced lower.
-MY 2010 Chevrolet Camaro
A fresh design with some retro touches. Cheap base engine (3.9L), powerful V8. Consumers will find that the well styled interior isn't just something to look at: materials will be top notch (cough, Mustang). A MY 2011 convertible will be soon to follow.
So, off the top of my head, there's where I woul start.