The M3 is well represented, but they're missing the original M, the car that kickstarted the whole thing:
I'd also love to see them add a bunch of cars that were in development, showed promise, or at least wild performance claims, but for various reasons were either produced in very limited numbers or not produced at all:
Italdesign Aztec. Conceived as a concept car, the roadster actually did make it into limited production with, I believe, a run of less than 50 cars. Powered by an Audi 5 cylinder turbo:
Yamaha OX99-11:
Tandem two seater supercar with a Yamaha F1 engine in the back. Designed during the boom in the early 1990s and I believe killed off by the market crash. Only 3 were built.
Vector W2:
Jerry Weigert's idea of a supercar, famously inspired by aircraft concepts in materials and dashboard design. Used an American V8, I can't remember which one, with added turbocharging, married to, um, a 3 speed automatic gearbox. Always fighting for funds, Jerry also made a number of bad business decisions, ran out of money more than once, and eventually lost control of his company. Only a handful were built. Jerry never stopped fighting to get his dream cars on to the market. Doug De Muro did a very detailed review on one of these a couple of years back.
Jiotto Caspita:
I can't actually remember much about this car except it had, for its time, some really nice curves on it. Japanese, if I recall. And I think it had some kind of link to a lingerie manufacturer? Another one-person's-dream that didn't quite make it.
Speaking of which, it would be great to see the return of the Dome Zero:
And one of my favourite cars from Gran Turismo 4, the Protomotors Spirra. Originally with a big V8, apparently this car sold in Korea and a handful of other markets for nearly a decade as the Oullim Spirra, powered by a series of V6s. Road tests are rare, as is, from what I can gather, the car itself, but those tests I could find were complimentary. I only know this car exists because of Gran Turismo.