Here are my picks for returning real-world circuits:
-Fuji Speedway (I'd add this for the Gr.2 event)
-Spa-Francorchamps (could be another Gr.1 Endurance event, IMO)
-Laguna Seca (could be ideal for another Gr.3 Endurance)
-Twin Ring Motegi
-Daytona (the infield course could be good for another Gr.3 Endurance, or Gr.1.)
-Indianapolis
-Monaco GP/Cote d'Azur (would be good for the F1500 event, or one of the Red Bull X-racer events)
-Silverstone (see Spa comments)
-Goodwood Hillclimb
-Red Bull Ring
-Sonoma Raceway (previously known as Infineon, and before that, Sears Point)
-Circuit de Valencia
-Ascari Race Resort
And here are my picks for returning original circuits:
-Matterhorn
-Eiger Norwand
-Deep Forest
-Trial Mountain
-Grand Valley
-Tokyo R246
-Rome
-New York City
-Seattle
-Toscana
-Apricot Hill
-Cape Ring
-El Capitan
-Grand Canyon
-Swiss Alps
-Tahiti Maze
-Complex String
-Sierra Circuit
I'm not a massive fan of Madrid nor London, personally, but for the sake of them being new (i.e. easier to spruce-up) and being interesting street circuits, I wouldn't mind them returning. I think they'd especially be good for races that use cars with lower horsepower, maybe Gr.4 at most. I'm also unsure about Midfield, since it seems a lot like Kyoto Miyabi and/or Dragon Trail Seaside are both pretty similar to Midfield in some ways. I'm also not sure about the really narrow courses from GT4 that were mostly for rallying (e.g. George V Paris), as much as I like them by themselves - I think they'd be far better as time attack courses like the Goodwood Hillclimb, and not as actual racing courses. And I have a feeling Opera Paris and Hong Kong would be really bad passing courses - like, they'd be the courses where the real winner is determined in the qualifying.
I'd also like to see the scapes from GT4/5/6 to eventually return, as well - I know that some have been remade in Sport already. I really wanna see Ronda again, for example, or Times Square.