first I agree with the NOT look like the Vette as I doubt that will translate well into a 4 DR sedanI'd add to your ideas.
Stretch Alpha even further, give it about 4" in wheelbase beyond Alpha+, and just make the next Impala RWD.
1) A series of SWB Alpha+ sedans: Impala SS, Regal, CTS, Commodore
2) A series of SWB Alpha+ coupes: Chevelle, Regal coupe, CTS coupe, Monaro
3) LWB (120" wb) Alpha+ sedan: Impala (maybe also a fleet-only Cadillac, distinct from the rest of the lineup, and also a new Statesman?)
I would not make the SS a Corvette-looking car. I would take the Impala's styling and evolve it more aggressively, but retain a very premium look.
I will add with the stories abound that the full size sedan is going the way of the DODO bird maybe GM will step up to the plate and create something more memorable then a good looking budget rent-a-car
the taurus is rumoured to be China ONLY next gen and the high rental-fleet numbers the impala family run VS Dodge having good retail sales with the charger