I want the RUFs (specifically the 3400S, CTR and CTR2), the Lancia Stratos Rally and Road cars (They are just too pretty to not have available for photo travel.), and a few others like the E-type Jag, the Caterham, the 22B (or any GC8 for that matter).
They really should be able to pull some data from the game and then easily come up with a prioritized list. Just take the cars that are used most (online, maybe offline as well) and and make that premium. Rinse and repeat.
The one car I would love to have as premium is the Caterham Seven Fireblade '02. Its such an amazing car and would be a great addition to the premium collection.
After that, I would like the Clio RS 2.0 16V '02, Subaru Impreza WRC '03, RUF CTR "Yellowbird" '87 and Audi Quattro '82.
I don't think that PD will upgrade some standards to premium... Have I lost any news? O.o
However
I would love to have premium:
Bugatti Veyron
Ruf BTR
Lancia Stratos
Lancia HF integrale
Lamborghini Countach
Lamborghini Nomad
Audi R8 LMS
Pagani Zonda LM
and all DTM cars...