As much customization that we do have in GT7 - insofar that I'd say we have the most options we've ever had in the series - I do still hope to see more tuned cars. Not only from tuners like RE-Amemiya, but also from Spoon, Mine's, Amuse, Opera, TommyKaira, and in-house tuners like TRD and Mugen. Plus at least a few more cars from Nismo, like the R33 Skyline GT-R LM Road Going Version and the 270R. Heck, maybe even some cars that mention Autech and SARD. And more non-Japanese tuning firms, too, like how I'd love to see the Canada-based HPA Motorsports return with their Volkswagen Golf and Audi TT.
Speaking of Nismo, I also wonder if it could be worth seeing vehicles that would be sold under Nissan, but have the Nismo branding, much like the GT-R Nismo '17. I'd totally be down for say, the Nismo version of the Sentra, or even the Juke Nismo. (Speaking of the Sentra, I'd wager the SE-R based on the fifth-gen model would be splendid as well.)
EDIT: I might've also already mentioned this one, but now that AMG is its own brand, their take on the Mitsubishi Galant would be amazing to see in GT.