No, it's not that I don't want 90s JDM cars in the UCD - as I do think that's where they belong - but I think there should be a wider variety of cars in the UCD in addition to that. Stuff that's older, but not rare. So like I mentioned, the BMW M3 E46 would go there, but not the BMW M3 E30 Sport Evolution, since the latter was a much rarer variant of the M3 E30. As another user mentioned, I could see something like the first-gen Ford GT being in there, too, but not some rare model where less than a thousand were made, such as the McLaren F1, the BMW 507, the Nissan Skyline GT-R C110, or the Lamborghini Miura P400 Prototype.
Honestly, if I had to pick a number, I'd have any given car in the UCD have a minimum real-world production figure of 2,000 units. Anything that had less than 2,000 units produced would be buyable from the dealership, whether from the main showroom, a historic showroom like in GT4, or something similar.
EDIT: On another note, with so many race cars being available, I wonder if each manufacturer should have a separate showroom for race cars? That said, I also wonder if having a separate showroom for VGTs like in GT Sport is necessary as well. I'd personally only have showrooms that can have a healthy population of cars in each of them, so perhaps I wouldn't have a separate VGT showroom, but replace it with a showroom for racecars. But I digress.