A quick shortlist...
Land Rover/Volvo/Jaguar/Aston Martin: I put Ford's Premium Auto Group all together. From Aston Martin, the new V8 Vantage and Vanquish S would be nice; maybe a classic or two like a DB5 or a Zagato model. Can't really think of anything from Jag, other than maybe one of the classic D-Type racers or an XJR sedan. Volvo needs the S60R, as well as the S40 T5, and perhaps an 850R(or T5 R) wagon. Land Rover can get the previous model Range Rover 4.6 HSE, the new Range Rover Sport(really based on the LR3), and maybe a really old Range Rover tuned by Overfinch.
Rolls Royce/Bentley: Some of these would be cool. Maybe a couple of the more traditional models, then the newest models done by their new owners, BMW and VW, respectively. From Bentley, an Arnage and a Continental GT would be nice for starters.
Porsche: IMHO, not really a must. It's hard to get licensing, so RUF fits the bill for me.
Audi/Volkswagen/Lamborghini/SEAT: The Audi S8 would be cool, as would the R9 911 rival and the diesel R10 Le Mans car. How about the Scirocco or the Corrado? I don't know what to do about Lamborghini, but the Audi R9 should offer a similar experience to a Gallardo, since they will use similar platforms. Perhaps more Seats could be used; even though they are reskinned or rebadged VW/Audis, maybe PD can leave gaps in the VW/Audi lineups so that Seat can have a couple more cars.
Mercedes: I don't really know what to add here. They're pretty well represented in Gran Turismo, imho.
Fiat/Ferrari/Maserati/Alfa Romeo Maybe some of the tuned Ferraris from Koenig could be added. If it is impossible to get Ferrari, maybe we could get Maserati as a compromise. I'd love to see the new Coupes and Quattroporte, as well as older cars like the Ghibli Cup. The rare and brutish Alfa SZ would be a cool addition.
Citroën/Peugeot: Ah, the PSA giants. I don't know much about them, but they seemed to have a decent representation in GT4.
Renault: The RenaultSport MKII Clio(MKIII should be out in time as well) and Megane are a must. The MKI Clio williams is a hot hatch legend, and would be nice to see.
Honda/Acura: They seem to be okay. Maybe add the Honda Legend/Acura RL, and maybe(a big maybe) the Ridgeline.
Nissan/Datsun/Infiniti: Pulsar GTi-R, S12 Gazelle, Micra R, Altima SE-R, Sentra SE-R(B13 and B15), Cedric/Gloria, Fuga 450GT, Cima. Infiniti Triant Concept would be cool, too. Additional NISMO specials are more than welcome.
*Additional note* Less GT-R variants, please. Maybe two of each(V-Specs and N1s??) and leave it at that.
Toyota/Lexus/Scion: AE86 Coupes!! How about a Trueno hatch and Levin coupe? The older rwd Starlet! Scion tC. Can't think of much else.
Subaru: The 22B was a nice start, but how about some of the limited edition STi cars? The S202, S204 and S204 are examples. Don't need to have them all, though. How about the basic WRX, which can be a cheaper, less sophistcated alternative to the STi.
Daihatsu: Can't think of anything I want to see.
Kia/Hyundai: They've got some nice concepts. Maybe more of them.
General Motors/Saab/Daewoo/Suzuki/Opel/Vauxhall/Holden: The Vauxhall Astra VXR, and possibly the Corsa and Vectra VXRs as well. Maybe the Zafira VXR minivan for kicks. The Cadillac V-series models would be cool. How about a final edition Camaro SS, and a Trans Am WS6? The old Pontiac Rageous concept would be cool, too. The Holden Ute is a must. The C6 Corvette and Z06 model would be nice too. The Cobalt SS and Solstice GXP are welcome as well. The old Bellaires(Nomad included) would be interesting, too.
Ford Motor Company: The new Focus ST, the Sport Trac Adrenalin(why didn't they just make another Lightning??), Contour SVT, '03 Cobra SVT(with the blower), Shelby Mustang GT500(I'm sure it'll be in there), recent Mercury Marauder, Lincoln LS V8 and Mark VIII. Older rotaries from Mazda like the RX-3 would be cool.
Chrysler Corporation: Some SRT models are welcome. How about some of the amazing concepts that never made full-scale production? The Razor, Copperhead, Tomahawk and Venom come to mind.
BMW: I fully expect to see the E90 M3 and the new M6. The E39 M5 was an amazing vehicle, and really deserves to be in the game. The 130i Sport is pretty cool.
Lotus: The S2 Exige, though I suspect that's a given. How about the limited edition supercharged 240R edition as well? An original Seven or Eleven would be sweet, too. Maybe the original Seven could be an alternative to the Caterham(a Lotus Seven replica), which could not be used in races.
TVR: The Typhon and Sagaris, please. The original Griffith would be cool, too.
More tuners would be cool, too. More on that later.