As a fan of both Pontiac and Plymouth it's a shame there are only 2 cars in game for each, so here is 4 cars that could bolster each brands ranks.
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Plymouth Prowler
A unique retro-styled factory hot-rod, the Prowler was the test bed for advanced aluminum forming processes. This was one of the last new Plymouth vehicles released, and one of the few that wasn't just a cheaper Dodge.
Plymouth Roadrunner
In an era when muscle cars were getting heavier and more plushy optioned, Plymouth went back to basics with the iconic Roadrunner, stripped back to bare necessities and powered by the baddest motors in the Mopar fleet.
Plymouth Voyager
While a pretty boring car as far as sporting goes, the Voyager and Caravan duo are unmistakebly key players in the evolution of American automobile culture, kick-starting a brand new automotive segment, of which the brand would be at the forefront for decades. Also in later years it got a spicy Turbo model which could be fun.
Plymouth Duster
The result of a skunk works operation to create a sporty version of the pedestrian Valiant, the Duster proved a smash hit despite big papa Chrysler's desire for Plymouth to focus on boring passenger malaise. With a potent 340 under the hood, this car would easily hang with the best of GT7's muscle car roster.
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Pontiac GTO(VZ)
A controversial car when new, with Holden coming back to GT seemingly unlikely, the next best thing would be the Pontiac rebadge. Despite being somewhat unloved in their day, these late-model GTO's have become modern classics, relatively affordable and easily moddable, and GT wouldn't be any different
Pontiac Firebird Trans Am 20th Anniversary
While the standard GTA would be the straightforward option, why be normal when you can be different? In 1989 for the 20th Anniversary of the Pontiac Firebird General Motors decided to drag back out the Turbo 3.8 from the old Grand National and give it the once over, creating the fastest accelerating American road car of the era.
Pontiac Fiero GT
Despite a rough launch and subsequent bad reputation, by the end of it's lifespan the Fiero was a potent machine, especially in GT guise powered by a punchy V6. This car deserves a place in GT as a worthy rival to the MR-2.
Pontiac Can Am
An obscure piece of American motoring, the Pontiac Can-Am was an attempt to bring Muscle Car flair to the slow selling LeMans. Unlike many other American cars of the time which were all show, no go, the Can-Am featured the engine from the contemporary Trans-Am, rated at 200 HP (which is only 10 off the stated power of that years Corvette.) Unfortunately it was canned after one year as GM brass thought each Can-Am sold took away from the Grand Prix, which was having record sales. A shame too, but we could be able to experience this car in GT if it were to come.