-I want to see the return of the manufacturer events like in GT4. I think that'd also be a good idea for the UX, instead of having those same events clutter the menus for the more "general" events, like Beginner/Amateur/Professional.
-I think GT3 had the perfect length for endurance races. In real-world endurances that last over 12h, it's never a single driver doing the whole thing, anyway. But in GT3, I think they weren't too short nor too long.
-I also want to see a blend of classic GT events as well as new ones. I could see Like the Wind truly returning, being a Gr.1 event set at Special Stage Route X, Daytona Speedway, Indianapolis Speedway, and Blue Moon Bay.
-Finally, I think Red Bull should be its own brand in Brand Central, and this is where you could buy all the Red Bull cars (X2010, X2011, X2014, X2014 Fan Car, X2014 Jr. and X2019 Competition), and there could even be a manufacturers event set at the Red Bull Ring, where you drive the X2014 Jr.
EDIT: I also unfortunately don't foresee the Nike One, the Ferrari F2007 and F10, the Lotus 98T, nor the Mercedes-AMG W08 returning, but if they did, that'd be fantastic, alongside the other Senna cars, the LH Edition Mercedes VGT, and other "Special Guest" cars like the Citroen C4 WRC, the Jeff Gordon NASCAR, the Mario Andretti Hudson, and the Vettel trim for the X2010.
EDIT2: I wouldn't mind the used car dealership being gone, and I really do prefer GT6 and GTS' approach to making all cars buyable - but I did sort of like the voucher system from GT5, so you could get a car later if you want, and also pick out the color then, too, instead of being put on the spot to pick a color. I also like the idea of the tuner village returning, too, but I also like the idea of in-house tuners (and those respective models) being accessible via those manufacturers, e.g. TRD and GR via Toyota, or Nismo via Nissan.
Yet another thing I'd love to see is a revived RM system, except it'd be how you outfit a car for one of the major groups, like Gr.4, Gr.B, or Gr.3, and this would include the road-car homologation specials. So you could use the Alfa Romeo 4C to get the Gr.4 car, but you'd need the Gr.3 road car to get the Gr.3 racecar. But you could alternatively take the Hyundai Genesis Coupe, and either turn it into the Gr.4 car or the Gr.3 car. Same thing with the WRX STI, except you'd need the Gr.B Road Car for the Gr.B rally car.