I think I want to see GT7, but with a successor to Sport Mode. There could be new classes, like a Gr.1V for the older prototypes and Gr.3V for older grand tourers, and plenty of cars that could be added to the other groups. For example, even if the new Lamborghini VGT ends up in Gr.1, I'd love to also see the older Countach QVX Group C car as a potential Gr.1 alternative. There could also be more online championships beyond the FIAGTCs and Supra Cup, each with interesting regulations. Imagine various GT campaign events, like Boxer Spirit, being represented as Daily/Weekly races, or even an entire championship?
There's just a lot that could happen, given how long PD has likely been working on this, and how much they've expanded. There's a lot of potential to expand on stuff seen in GT Sport, like the companion app, the in-game social network it has, or the livery editor. That's also before we get into stuff that could return from older games, like the track creator.
On the other hand, I don't know if the Lewis Hamilton content will return, as I don't know what sort of licensing costs are associated with that - including the ability to have the W08, and the LH Edition Mercedes-Benz VGT from the DLC. In an ideal world, I'd love to see all the "special guest" cars return, between the W08, and the cars used by Jeff Gordon, Sebastien Loeb, and the Sebastian Vettel X2010. That also would include the Senna content, but as I said, I really don't know how much all this would cost in licensing. But the cars themselves I think would be enough for me. And the two Ferrari F1 cars from GT5 would be great, too.
My personal bet? I think some sort of revived rally/hillclimb component is next, given that the only real-world Gr.B car is a Pikes Peak car, PD has the rights to the course itself, and we already had a hillclimb course with Goodwood, which I'd also like to see return. I could see it working well as an evolved form of the Sport Mode time trials, where you could be tasked with a relatively random car (or category) to use on a given course, with prizes depending on your position within your region.
But overall, I don't think you hire the positions that PD has been searching for (like the UI designer from some Ridge Racer games, or a mobile app developer) for a mere PS5-enhanced version of GT Sport. I think GT7 is next.