I personally don't mind the current cover design we've been seeing, even if it's also likely to just be a mock-up. If it truly is a mock-up, I'd bet on the Corvette C8 being on the cover, since we GT5 had the SLS-AMG, GT6 had the Corvette C7, and GTS had the AMG-GT. But I could also see it being some sort of car that has that grand tourer silhouette - i.e. a GT car - like the Nissan 400Z that the OP mentions. That said, I think it'd be too late to do that, considering how long game development in general can take, and when the new Z was revealed.
But I think the possible mock-up cover would be fine, given that it features two automakers that currently have a pretty close relationship with GT at the moment, namely Mazda and Porsche. (At least, I think that's the Porsche VGT on the cover.) Alternatively, I'd be content with a Toyota GR Supra being on the cover, too.