The Lost Tracks
Nürburgring Südschleife
AVUS Berlin
Riverside
Daytona Beach Roadcourse
Circuit de Reims-Guex
Circuit de Montjuic
Targa Florio
Mille Miglia
Sadly the Targa Florio isn't very likely being nearly 80km in length and I would think the Mille Miglia would be impossible at 1000 miles, but agree it would be nice to get some lost / classic tracks - especially ones which bring an element of that targa feeling - Circuito da Boavista, Portugal might bring that feeling with a couple of long straights and a winding section through the houses - it was used by F1 in 1958 and 1960.
Longford, Tasmania could be great with the classic cars - the nearest equivalent we have is Bannochbrae. Longford was used for the Tasman series and saloon racing and would be a big challenge.
I don't think I would want Avus as it was basically 5 miles of highway with a loop at each end, but I would definitely want Reims, Riverside Raceway and Circuit de Montjuic.
I would also add the following lost circuits to my wishlist - Aintree - former home of the British GP - much of the track still remains, including the club layout, so it could be drone scanned.
Crystal Palace, London was a wonderful circuit set in beautiful parkland which closed in about 1970 - Mike Hailwood holds the lap record. It was great for single seaters and saloons in the 1950's and 1960's and would be great in game as we wouldn't need to worry about the dangers of unguarded trees and other hazards.
Dundrod, Ireland would be nice - it was used for the RAC Tourist Trophy and the Ulster F1 Trophy - its 7.4 miles had a great mix of straights and both fast and slow corners although it was dangerously narrow for cars so would need widening slightly for good gaming. Mike Hawthorn was the master of this track - it would be lovely if we got a D type Jaguar so I could pretend to be Mike!
Other great lost circuits could include Greenwood Roadway, USA - rumoured to be the inspiration for the layout of Deep Forest in Gran Turismo. Sadly it was only open for 3 years in the 1960's, but held some important races including USRRC.
A couple of favourite tracks of Stirling Moss - Clermont Ferrand (also known as Charade) - a sort of mini Nordschleife and sometimes home to the French F1 GP it had a great mix of corners . Another great drivers circuit was Bridgehampton, USA sadly now lost under a housing development and golf course.
Another possibility is Stardust International Raceway, Las Vegas, USA - it was a 3 mile circuit which hosted Can Am, Trans Am and USAC Champ Car races - sadly now lost under a housing development.
Finally how about Nassau, Bahamas - used for the Nassau Speed Weeks, which always used to attract a mouth watering field of cars!
Below - Riverside International Raceway in 1965 driven by Skip Scott:
Below Bridgehampton in 1965 (includes plenty of cars I would like to see added to the game):
Below Nassau in 1964: