My 2 cents.....
Chevy - subcompact, compact, mid-size, compact CUV, full-size CUV, minivan, Corvette, Camaro (can't kill these two iconic names for the brand). Price them slightly under the Japanese to make them attractive (does not apply to Corvette and Camaro).
Caddy - compact, mid-zise, full-size RWD, compact SUV, full-size SUV
Buick - No other value but the image in China. I'm not sure if they stop selling it in NA would be able to preserve the brand image in China (an American brand that is not sold in America). Give it a mid-size/full-size car and a full-size CUV. If China is not an issue, kill it.
GMC - all the trucks, truck-based SUVs, vans and other commercial vehicles. Trim the production to reflect the demand. Kill some old models in the process.
Pontiac - keep it for now as the RWD/sport division. Keep G8 and Solstice, but kill the G5 and G6.
Saturn - Ideally I'd say convert it to Opel as the name doesn't have any strong resonance anyway (besides it sounds like crap and it reminds of Ion) and bring Corsa, Astra, Insignia, Zafira. Get rid of Outlook and Sky. Rename Vue to Antara (and make it lose some weight in the process). Build the cars in N/A, trim the features and keep the price in line or slightly higher than Honda/Toyota/Mazda/Nissan, but cheaper than the VW.
Hummer - keep the H2 for posers if it's worth (I don't know that), but scrap the H3 as it doesn't have a place in the marketplace. Alternatively, kill it.
Saab: Put it up for sale.