For me, it'll be based off of the original Gran Turismo games where there is an Arcade Mode and a Simulation Mode. Arcade Mode gives you access to:
>all cars
>all upgrades
>all tracks
>adjustable track settings
>adjustable difficulty
Arcade Mode also has similar physics to RR4's predecessor.
Simulation Mode converts to the game to more of a simulator than an arcade racer. Sim Mode gives you access to:
>all cars, upgrades, and tracks must be unlocked
>true-to-life physics
>cup series (includes regular racing and championships)
There will also be 3 main categories, which are split into multiple subcategories. These are:
>Performance Cars
>Rally Cars
>High-End Race Cars
There will be lots of iconic tracks, like:
>Nürburgring
>Laguna Seca
>Daytona
>Brands Hatch
>Circuit of the Americas
>Watkins Glen
and many more.

That's all I have in my mind right now. If you controlled RR4, what would be in it?