At least 2 or 3 63-AMG's, I don't care which ones, they all sound awesome. The SLS-AMG should be in too, sofar I don't think there's a PC game in development that has it, whereas it already made its debute by being the covercar for GT5. For the SUV's I wouldn't mind the latest G55, but I'd prefer an ML63 AMG. I also would love to see the C111 IID or C112 in the game, as I think the games that had it can be counted on half a hand. And ofcourse CLK-GTR, perhaps even as roadster :D
But my main hope, especially for Mercedesses, is that if they choose cars that are currently in production, that they some more recent versions. The CLS55 and CLK55 were already outdated when TDU got released since they had facelifts and the 63's, and the SL65 was the very first generation.
Other than MB would personally have to be a Diablo and the EB110 GT (not the SS, its spokerims are ugly and simple). And if there are nice areas like where that Concours d'Elegance are always held, I think some unaffortable classics, oldtimers and superrare exotics should be in.
I also hope that the models will be more realistic than in TDU. I mean, they weren't really horrible, but lots of proportions were not correct, such as the width of the Pontiac GTO, the wheelarch sides of the CLK-GTR, and some more I forgot. And the bug of wheels changing sizes when tuning should be gone :P