1. Chevrolet, which would handle the family car market plus the Corvette.
2. Cadillac, which would handle the luxury car market.
3. GMC Trucks, which would handle the light truck and SUV markets.
4. Isuzu Motors, which would handle light trucks and heavy trucks made in Japan.
5. A combined Opel/Vauxhall operation, which would handle models sold for European (and selected world) markets.
By the way, with interest in more fuel-efficient vehicles in the USA, we may see a number of Opel models make it to the USA. The current Zafira and Meriva models need very little modification to make them US-market compliant, and there is some talk that we may see the Meriva sold in the USA, where the car will be built in Korea by Daewoo Motors (which GM now has major controlling interest in).
GM sold off its interest in Isuzu (who is about ready to go under BTW)
The current Chevy Malibu is basically a US built Opel...similar chassis design
They also have a decent operation in Australia, Holden (which still makes big sedans that are loved by Aussies)
I have a feeling that Toyota and Honda are ignoring the Korean auto makers..primarily Hyundai..that are going after their markets.