Cadillac - Just not smart to give a baby car to this division in any way. It would dilute the brand just like the 1 series and A class have diluted BMW and Mercedes.
Buick - Now I know some of you might think I'm crazy here, but hear me out. This size of car for Buick would be a major hit in their biggest market, China, plus it would bring in a whole new group of people into the dealership here in the States. If done right it could fit in perfect with Buicks new direction. This would most likley need to be front wheel drive to fit in with the rest of the lineup.
Chevrolet - No "premium" small car should be done here. A low cost high quality small car needs to be done instead.
Saab - Wow could they have fun introducing a small car that had resemblence to the Saabs of the 60s. I have no doubt people would be all over it. It would also help Saab with their world wide sales #s.
Saturn - If Opel has it, so then should Saturn. The Corsa seems small enough and cheap enough for now. Maybe adding some diesels from Opel would make more sense than smaller cars.
Pontiac - Everytime a magazine gets a hold of the Mini they fall in love with it's sporty small style. A super small rear wheel drive car would fit perfect with Pontiac just like it does with BMW. If they want Pontiac to be a lower cost BMW, then this would be perfect.
RECAP; Buick, Saab and Pontiac are the only brands that should be considered for such a car. Saab and Buick could easily share the chassis, but Pontiac would need something totally different.