No. Just no.
Radical, but completely devoid of good business logic. You are not looking at the industry from a profit standpoint. GMC is pure profit, therefore don't f*** with it. Chevrolet is about to sell 3 million, therefore it doesn't need internal competition. Chevrolet competes with Toyota. Why should Saturn? I'll answer that: it shouldn't. Let Saturn specifically target upscale brands like Acura, VW, and Mercury, and if the product is good it will get conquest buyers from Toyota and Honda who are trading UP, not the same people in the market for a second or third Camcord.
The problem is that you are focusing on competitng brands instead of market segments. Pontiac will likely be the part of the B-P-GMC trio that caters to the "youth vehicles" i.e. cheap, fun to drive, sporty, but not too well-appointed. If you want to think in terms of competing brands, then somewhere between Plymouth, Dodge, Scion and Mazda. Yes, I said Plymouth because those used Plymouths are still being sold.
Buick is currently in a bit of an identity crisis. I don't think GM knows what it wants to do with it, and I see that as a problem. Personally, I think GM should take a two-pronged approach to the luxury market and sell Buicks and Cadillacs in the same dealerships, but different showrooms. The Buicks should target the Lexus/Jaguar luxury mentality of plushness and crypt-like quiet, while Cadillac should focus on BMW, Mercedes and Infiniti as far as offering more sporting cars with firmer suspensions. Cadillac takes more radical, flashy styling while Buick is more subdued and elegant. I think it could work if positioned right, though I doubt GM would be capable of that.