Just remember this-

One of the big mistakes GM made with the F-Car is that they had a "low-volume" vehicle built on a unique platform at a plant that built only the F-car line.

What GM has to do is adopt some of Holden's production principles, that is, you take a single platform and build a whole bunch of different vehicles off of it with the appropriate modifications. In GM North America's case they should design a low-cost platform that can handle RWD and AWD with V8 engines.

Another thing to consider is this -

If you want to build a few niche vehicles one thing you can do is not only share platforms, but share as much of the electrical architecture and electrical components as you can. You can also share other components that are not critical to the brand, image, and performance of each of these vehicles (dome lights, fuel filler caps, HVAC control heads, hood release levers and cables, some electrical switches, etc.). The more sharing that you do among these niche vehicles, the closer you come to having a single, high-volume vehicle line, and you can have higher utilization of plant capacity as well.

 

Well said hotrodtodd74, thats why many think(thought) that if a 5th gen does appear it may come off the same platform of the new CTS (Sigma Platform), which is somewhat expensive, but like you said the more cars you can build using the same parts the cheaper it is. I certainly wouldn't mind having a few caddy parts in a camaro.