Hopefully UPDATED "old cars" (no more standards please) will be the path PD pursues.
I think we already begun to see that with the updated versions of some standard cars in GT6. And I don't just mean thing like the Lancia Stratos, but also cars like the Nissan R390 Road Car, where it was still a standard, but had notable improvement.
I think ideally, I'd love to see a large variety of cars, but maybe with some excessive models trimmed (such as the EU and US-spec versions of some JDM cars), and sorted into classes so that they could compete with each other. Perhaps, for example, a Retro-N300 class that includes the various JDM cars that were advertised as having around 275HP from the late-90s. Or maybe another class that could fit in historic vehicles, or the various tie-in vehicles we've already seen like the Andretti Hudson, the Jay Leno & Senna cars, etc.
And instead of being able to buy various parts, perhaps instead some tuning options would be available for certain cars, reflecting their real-world counterparts. Instead of say, upgrading the turbo or reducing the weight, you could take an N-Series car and upgrade it to a Gr.4 or Gr.3-spec version, where more tuning options could be unlocked, such as adjustment of downforce for cars that obtain a wing where they didn't have one in their stock form, and you could also tweak the gear ratios, ballast, etc. And this is all while you'd still be able to tweak the options of the N-Series cars that allow you to do so in reality, given how complex some road cars can be today.
To get back on-topic, I think to be realistic, I think if we see more Porsches, then my best guess would be ones made past 2009, right? Because that's the year of the oldest car in GTS confirmed so far. So that still means we could see stuff like the 918, or the Cayman - keep in mind that the Cayman and 911 have tons of variants, too. Though I'm not sure about the Macan, Cayenne, or Panamera, since there doesn't seem to be much otherwise confirmed in the GTS lineup. But if they say, announced the Alfa Romeo Stelvio would be included, or the Lamborghini Urus, then I'd think the Macan would be more likely to be in.