Interesting perspectives
Race cars only, it's a racing sim. The only reason street cars seem popular in AC is because there bundled with proper race cars. The reason I bought the DLCs was to get the race cars, not the street cars, I never drive the street cars no matter how exotic. I know GT3 and GTE cars are derived from street cars but so they are so far removed from the street car.
Road Cars Only, it's a driving sim. The only reason, race cars seem popular in rF2 is because they're bundled in packages of GT classes, that everyone can drive everywhere while getting wrecked at T1 at Monza...oh wait, no there is no stock-modern-Monza.
How can a rAcInG SiM not contain of Modern-Monza, when it has GT3? Anyway, losing track here.
The reason i bought the DLC in AC were the road cars, because they are the true experience, who cares about driving a GT3 amateur car in circles, when you can drift a 370Z sideways through each corner at Mugello? Whait...the 370Z is japanese pack content, isn't it? Of course nobody would prefer the boring GT3, when they could drive an exciting JDM car sideways!

(can contain of ironic mouthness)
But to be a bit more serious here, a little wishlist:
In terms of race cars...I still miss the actual Super GT or Super Formula in the physics focused sim titles. But the mentioned GT4 class really would be a nice thing to look forward to. These cars have more differences inbetween their class, than the GT3 cars and need a different driving approach.
In terms of road cars...NSX and Corvette overhaul would already be a major thumbs up from me. The NSX was really fun to drive at something like Oulton Park.
In terms of vintage cars...drivetrain/transmission overhaul + Lotus Elan S1
...i mean, look at this cutie...she's so happy to be on the road ^_^