Getting more fictional racecars would be quite the challenge. We get a lot of fictional GT racing cars because they're quite popular in other games, and all of the models in AC are from those games. To create one from scratch like a Giulia DTM, which is well thought-out and rendered, would require a concentrated effort from a sizeable team of people I bet.
A large problem is that most of the large AC modding teams are only concerned with developing real competition vehicles with established forms they can recreate and real data they can use. To make something like a fake GT3 car from scratch would require an immense amount of effort compared to just recreating something which exists already with usable data. You'd need to essentially "doctor" how the car would feel if it existed, but of course in sim racing people are pretty picky about that sort of topic so I'd imagine there would be some disagreement about how a fictional race car based on a real car should handle if one were to be developed.
If you look at Gran Turismo for example, there is a considerable amount of effort needed to design and model aero parts, interior bits, roll cages, etc. from scratch and placing it onto an already-existing car, while also making these parts make sense realistically and have them conform to the regulations of whatever class you're making a fake car for.
It would be cool, of course, but don't hold your breath.
www.racedepartment.com
