Many of the designs I like have already been discussed, but I would like to add something to the narrow truck suggestions. The Chevy C series has already been mentioned however something along the lines of the S-10, the Ford Ranger line, and similar vehicles by many other companies (particularly US and Asian companies it seems) should also be included. I know trucks wont be a focus of the game but they are rather popular in some circles. Personally, Id love to see the S-10 added for personal reasons but that type in general would be good in my opinion. Possibly some of the larger designs (F-250/350 series) if you are going to add fleet type contracts. I dont know if it would be worth it for just regular sales though as I think most are purchased for work type vehicles. I could be mistaken there though.
Basically, a few full sized and light trucks with the option of heavy duty if corporate orders will be in the game.
Also, throwbacks like the already mentioned PT Cruiser would be nice but dont forget about others like the Chevy HHR.
I agree with killrob that posting a ton of different designs would not be practical to implement, but what about doing a generic version of many of the classes (like listing the S-10 and Ranger class truck as one style). It would simplify their implementation in the game and also keep the in game interface from being cluttered with huge lists of designs that will probably have to have generic names for copyright purposes. This has probably already been gone over, though.