I'm going to cheat a little here and give two lists because I've always thought a "concept car edition" DLC package would be a very nice addition.
For the normal cars:
1. 1995 McLaren F1 LM (I never understood why no one does this car... )
2. 1971 Alfa Type 33 Stradale (Possibly one of the most beautiful cars ever)
3. 1991 Mazda 787B (Let's hope the rumors are true! The SOUND, oh the SOUND!)
4. 1969 McLaren M8B
5. 1987 Sauber C9 / 1988 Jaguar XJR-9 (Still haven't forgiven GT for giving these beauties with a standard model treatment!)
6. 1993 Dauer 962 (Possible workaround for the Porsche problem?)
7. 1963 Ferrari 250 GT SWB Lusso (Steve McQueen's)
8. 2012 Mercedes Benz G55 AMG (Not a huge fan of the current SUV selection, this would be my choice)
9. BMW 3.0 CSL "Bat" (I know we get the standard one, wish we could have the Racing ones)
10. 1959 Maerati Tipo 61 "Birdcage" (The 'true' Birdcage)
Concept Cars:
1. 1995 Renault Espace F1
2. 1995 Ford GT90 / 1996 Ford Indigo / 1991 BMW Nazca C2 (Cars of Need for Speed!)
3. 2011 BMW i8 Concept
4. 2005 Holden Efijy Concept
5. 2011 Jaguar CX-75 Concept (Turbine Sound!)
6. 1970 Bertone Stratos Zero
7. 2000 Audi Rosemeyer / 1991 Audi AVUS Concept
8. 2011 VW Concept L1 (Hypermilling!)
9. 1953,56,59 GM Firebird Concepts
10. 2005 Shelby GR-1 Concept