I'd say many of the fictional LM Race cars from the early PS1 and PS2 days, whether the Del Sol LM, the Camaro LM Edition, RX-7, FTO, Altezza etc.
From what the current car roster shows, is a distinct focus on either well heeled real world race cars, or modern fictional cars for GR.3/GR.4.
Now from the perspective of the games shifting projection of what it wants to be, the fictional race cars were very much included as a replacement for real world machines they couldn't get, of which is not as big of an issue now that PD is the top dog in the space and has full support of Sony's pockets to secure whatever they want.
BUT I feel these cars are the kinds of things that modern games SHOULD be doing more of: Using the fact they exist I in a digital medium to play around with ideas in a creative manner, and IMO it is as much a part of Gran Turismo's identity as the original circuits, and the fact they still occasionally put out "PD Specials" shows they still find value in these kinds of cars, with VGTs being an extension of that philosophy even if implemented in a questionable way.
Really the only recent "classic" Fictional Race cars are the Silvia and Civic Touring cars which were in the game at launch, and the Ford GT LM, the lone PS2 era model included, with almost every fictional race car since being some form of contemporary grouped racing car. This is a bit of a shame IMO, as even though we can technically build our own versions of Racing cars, it is nice to have a "baseline" vehicle to compare it to.
Now some of the PS3 era fictional race cars probably will make it back at some point, as they have proven they can update/overhaul old models from that era(aforementioned Silvia and Civic, as well as the Roadster Touring Car) but the PS2 era quality is such a drastic gulf that they would essentially be building the cars from scratch all over again, and knowing how long it takes PD to build a car, is probably juice not worth the squeeze. And the PS1 cars? Fugeddaboutit.
