Beyond the cars (and likely additional track), I'm just as curious in seeing what sort of miscellaneous changes and content could be coming. Stuff like QoL changes, potential expansion of the Sport Mode Time Trials, the HRE wheels - stuff like that. I'd say it's even more fun anticipating that, versus the car/track additions, since they feel comparatively harder to predict. Not that I ever would've seen these next cars coming, either, nor last month's RUF.
I mean, I feel like there's a lot of potential for GTS, including the addition of more race cars as some may've mentioned already, but I can't help but to think that a lot of things could be getting saved for GTS' successor in the next GT game. Stuff like a class for vintage prototypes, or the idea of making more automakers have more cars in each group. Ideally, I'd love to eventually see the Manufacturers' Series include more than just Gr.4 and Gr.3 races, and more specific selections of cars within a class, including the N-Series. Something more overall than just going between the following: all cars in a class being eligible, one-makes, or the very rare event for FF Gr.4 cars.
I get that trying to balance all the cars in a given N-Series class is a fool's errand, but what'd be wrong with having something like an N100 race that specifically only permits the kei cars?
I did have a hypothesis: maybe there aren't many race cars being added due to how the Sport Mode events are created. I already had the idea they were largely automated, and this could explain why it feels like there's not much variance in the general rules for the races, including variance in which cars are permitted. Perhaps the way the Sport Mode events were designed wasn't a factor when they added stuff like various Gr.1 and Gr.3 cars, as well as Gr.2 as a whole, but it could be that they're now using a more automated method to free up staff as they work on the next GT game, and as a result, this current method somehow doesn't accommodate the further addition of various racecars, at least other than those that may get added to Gr.X.