If they return, I'd imagine they'll return in a similar fashion to the Super GT500 cars, in that only a few of 'em will return. I think it could get too overwhelming for everyone involved (especially BoP changes) if every NASCAR vehicle returned. However, I could see a total of nine cars returning, since GT6 had cars from the 2010, 2011, and 2013 seasons, each with three automakers, with Toyota, Chevrolet, and Ford.
EDIT: If NASCAR (or a stand-in of some sort) were to appear in GT Sport or a future GT game, here's the list of courses from prior games that I'd personally pick as a "track playlist" of sorts, in no particular order:
Daytona Speedway
Northern Isle
Blue Moon Bay
Indianapolis Motor Speedway
Suzuka East
Special Stage Route X
Infineon Raceway - Stock Car Course (now known as Sonoma)
Twin Ring Motegi Super Speedway
I'd also pick the Super Speedway from GT3 as well as the Test Course, but I'm not 100% sure if they'd be made redundant or not by the other tracks here.