As far as tracks are concerned:
BRING BACK EL CAPITAN!!!
BRING BACK HONG KONG!!!
BRING BACK MIDFIELD!!! (if it's not in GT5)
BRING BACK VALENCIANA!!!
BRING BACK TEST COURSE!!!
BRING BACK LAS VEGAS DRAG STRIP!!!
Plus:
Grand Canyon
Chamonix (Original)
Cathedral Rocks
Smokey Mountain
Red Rock
Grundelwald
Costa di Amalfi
Citta di Aria
Fuji 80's
Fuji 2005 GT course
Pikes Peak
Just to name a few...
Plus some new ones:
Philip Island Raceway
Brno
Zandvoort
Surfer's Paradise
Road America
Dover International Speedway
Nashville Speedway
Nashville Road
Rockingham Speedway
Silverstone
And many more...
Cars:
Polyphony 001
Polyphony 002
Aston Martin DB7 Volante
Aston Martin Rapide
'67 Chevrolet Corvette Stingray 427
'97 Chevrolet Camaro 30th Anniversary
Dodge Concept Car (and RM)
Dodge Viper GTS (with the Blue/ White Stripes), RT/10
Mazda Eunos Cosmo 20B/13B
Mazda RX-7 A spec
Mazda RX-3
Mazda RX-9 Concept
Subaru SVX (any)
Toyota Chaser (all)
TVR Cerbera LM
Nissan Silvia LM
Mitsibushi FTO LM (GT1)
Mitsibushi GTO LM
Honda NSX LM (GT1)
Tom's Angel TO1
Tommykaira ZZIII
Aston Martin DB6
More Ruf's (love them)
Audi TT LM
BMW 850i
BMW 750i
BMW M5 (E28, E34, E39, and F10)
BMW M3 (E30, E36, plus racing models)
BMW M1( plus race car)
Lancia Beta Race Car
Citroen Saxo 1.6 VTS
Peugeot 205 GTi
VW Golf Rallye G60
VW Golf W12
Ford GT40 Road Car (GT3)
90's Ford Mustangs
Ford Escort RS1600, RS1800, RS2000, XR3i, RS Cosworth
Ford Fiesta (Rally Car)
Ford Fiesta XR2
Ford Mondeo ST220
Mugen Accord
Mugen Civic Type R
Mugen Integra Type R
Honda S2000 GT1
Jaguar XJR 15
Jaguar XJR
Jaguar XKR-S
Lancia Delta HF Integrale Collezione
Lister Storm V12 Road Car
Lotus Elan S2 90
Mazda AZ1 Aspec
Mazda MX5 A,B, and Cspec
Mercedes C43 AMG
Mercedes SL73 AMG
Mercedes E55 AMG
Mercedes CLR
Caterhams
Nissan Pulsar GTiR
Plymouth GTX
Plymouth Roadrunner
Renault Espace F1
Shelby GT500
Shelby Cobra Daytona
Tommykaira M13 Stage 2
Tommykaira M30 (Mercedes 190)
Tommykaira M30 GTS
Tommykaira M30 GTSR
Tommykaira R '95 and '99
Vector M12
Weigert Vector M8 Twin Turbo
Vector M12 LM
Venturi's
and I think that's about it for the moment...