There are many cars and tracks I'd love to see included in future DLC, possibly too many to mention, though a fair number have already featured in previous GT games. The majority being from GT2 with a few tracks / cars from GT4 also, here's an example of some..
Aston Martin DB6
BMW 3 Series
BMW 7 Series
Chevrolet Camaro Z28 30th Anniversary '96
Ford Mondeo Ghia X 2.5i 24V
FPV GT
Mitsubishi Galant VR-4
Nissan Pulsar GTi-R '91
Vauxhall Astra SRi 2.0i 16V
Vauxhall Vectra GSi 2.5 V6
Plymouth GTX
Plymouth Musclecar (I believe it was GT2's generic Roadrunner, probably due to licensing)
Renault Laguna V6
Renault Espace F1 [R]
Shelby GT500 '68
Subaru SVX S4 '95 (AKA "Alycone" I think)
Vector M12
Vector Wiegert Vector W8 Twin Turbo
Venturi Atlantique 300
Venturi Atlantique 400GT
Track wise (including all variations of..):
Seattle
Pikes Peak
Midfield Raceway
Apricot Hill
Red Rock Valley
Tahiti
Grindelwald
Fuji 80's
Infineon Raceway
Twin Ring Montegi
El Capitan
Paris
.. and many more I'm sure
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If I were to think up some actual DLC packs I might suggest something like a vintage themed back, especially after seeing that video in the U-Toob section of the 1967 Nurburgring. Something like a 60s Nurplepurple Ring and another similar era track with some period race cars and road cars.
Another one that springs to mind from my love of the 70s is perhaps a 70s American themed pack, it could include the Seattle tracks and one set in another city (perhaps a San Francisco or New York). Car wise I'd love to see some big 70s coupes and sedans like the Cadillac Eldorado, Chevrolet Caprice, Buick Riviera, Oldsmobile Delta 88 Royale.. the kind of cars you'd see in episodes of The Rockford Files, STreets of San Francisco and such
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Stopping now before I start to pick up too much of a head of steam!