I think if GM sticks to what they claim their plans are for the Buick and Pontiac brands are, then it makes sense to have them both in one showroom.

Buick is supposed to be a a reasonably luxurious car that's not up there with Cadillac and is more competing with the likes of the Toyota Avalon, Lexus ES330, Acura TL and other relatively large entry-luxury cars. They could have a passenger car of that size, let's say a LeSabre, then a larger passenger car say of Chrysler 300 or Ford Five Hundred proportions, let's say a Park Avenue. Probably keep both of these fwd with an awd option. Add a true 4-seat large rwd coupe (Riviera, anyone?) and a nice convertible version of that coupe, and there's your four Buick models. Maaaaybe keep a minivan and crossover SUV around since no such products exist in the GMC lineup and they certainly don't fit with the proposed image for Pontiac, existence of the Pontiac Torrent Chevy Equinox clone notwithstanding.

Pontiac is supposed to be sporty and with a bit of flair but also reasonably practical. Their existing models at least in theory make a certain degree of sense but their execution is rather poor. They can continue to have a G6-sized vehicle but it would need to be more sporty and better screwed together. Let's say the Volvo S60 would be a good example of what to target, although with a bit more sport to the Pontiac competitor and obviously a lower price. They're planning a G6 coupe and hardtop convertible for very soon. Make the same improvements across the G6 line and you have three models right there. The GTO works as a larger coupe with a V8 and if made more interesting than the current blandmobile could be good as their big muscle car. Optionally, but not necessarily, they could have a smaller muscle car (Firebird?) to compete with the Mustang. The new Mustang has shown that a muscle car can still sell in good numbers if done right. Maybe make convertible versions of the GTO, Firebird or both. Finish up the lineup with a larger sedan somewhere in between the two hypothetical Buick sedans (Bonneville?) that's more overtly sporting, perhaps rwd to directly compete with the Chrysler 300.

GMC is pretty solid as is.

Combining the three brands covers pretty much every segment, and while both Buick and Pontiac would have sedans, coupes and convertibles they would be differentiated enough that they would complement, rather than cannibalize each other. Customer comes in lookin at a GTO but it's too loud and doesn't ride nice enough? Take a look at this Riviera. Customer comes in lookin at a LeSabre but it's not sporty enough? Step this way and check out a Bonneville. Customer wants a nice convertible but can't justify the price of a Riviera? Whaddaya know, this G6 is pretty nice too and it's a lot more affordable. Customer comes in on a GMC Envoy but it's too trucky? Well here's this Buick Rendezvous. The three brands combined would be not really all that much more to sell than what Toyota has on its lots. And those big megadealers would be able to consolidate some of their multiple outlets into single outlets, reducing their overhead and giving the sales team more ways to keep customers in the same showroom.

Hey, maybe GM should hire me...