I agree with the AeroVette and Furai, awesome machines. Either of those would get my vote from their list. My additions would be:
Saab aeroX, I love its looks and the cool canopy design. Perhaps it would be too extreme to sell decently, but I think every manufacturer should sell something that's way out of the book at least once.
C111-2, the original one with the wide tires and wankelengine and not that half historically correct V8-powered thing they're driving outside on a monthly base. I think this car would have made a nice couple with the M1.
C112, would have fitted nice in the group of 90's supercars, being pretty different (slower and very computerized). Used a lot of components from existing cars so it still beats me why they didn't see a future with the C112. To complete the German contribution to that whole class of 90's awesomeness, I'd get an Audi Avus as well.
My own C114!.....j/k......nah not really, I want it.
Some more cars, like the Kia GT4 Stinger, Opel GTC Concept, Holden Efijy and Holden Coupe 60.