OK, looking at Pontiac's current lineup:
Vibe -> Rezzo (a Daewoo; based on Nubira and "to-be" Cobalt)
G5 -> Astra coupe? (Saturn gets the hatch and sedan)
G6 -> keep; a reskin or two?
Grand Prix -> Commodore
Solstice -> keep
GTO -> Monaro
Torrent -> Adventura
SV6 -> kill
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Continuing on that, heh...
Gamma - Corsa (Opel/Vauxhall, Saturn), Gentra (Daewoo, Chevy Aveo NA and EU)
Delta - Astra (Opel/Vauxhall, Saturn hatch/sedan, Pontiac coupe), Nubira (Daewoo, Chevy Cobalt NA and EU), Rezzo (Daewoo, Chevy EU, Pontiac)
Epsilon - Vectra (Opel/Vauxhall, Saturn AURA), Tosca (Daewoo, Chevy Malibu), LaCrosse (Buick NA and China), 9-3 (Saab worldwide)
Theta - Antara (Opel/Vauxhall, Saturn VUE) Captiva (Daewoo, Chevy NA and EU)
Zeta - Camaro (Chevy NA), Lucerne (Buick NA and China), Monaro (Holden, Pontiac GTO), Commodore (Holden, Pontiac Grand Prix), Statesman (Holden, Chevy Impala NA)
The G6, HHR, and other NA-only cars would have to be refreshed on their current platforms.
I suppose Holden and Chevy SA/Middle East can pick and choose between Opel and Daewoo Gammas, Deltas, and Epsilons, depending on local tastes, and vice versa (Opel and Daewoo getting a few Holdens).
So by sharing... there will no longer be US-exclusive Cobalts, Malibus, Equinoxes, G5s, Vibes, Torrents, Grand Prixs, Impalas, IONs, and AURAs, which should help cut development costs, I think, and improve the US lineup by eliminating redundancy (G5 =/= Cobalt, Equinox =/= Torrent) and offering more RWD (Impala, Grand Prix, Torrent, Lucerne).