More Modern/ish cars, made with the same quality all other cars that ship with the game, the only one we have is the Sunburst, and I remember back when the most modern car was the Grand Marshal.
This isn't my most desired thing to have in-game, but I think this should be in (this was in Rigs-of-Rods). I don't know what to call it, and the only way I can explain it is you click a part of your vehicle, drag, and it pulls the vehicle in the direction your cursor goes... and it looks like so.... It comes in handy when you have flipped your still drive-able vehicle, and getting it back on its wheels with minimal damage. EDIT: Also, since the addition of your engine shutting off when you go under water, if you can get the vehicle out of the water there should be a way to turn it back on... or if there is a way and I just don't know how I wish someone would tell me...
I'd have to say the Click and drag thing as mentioned above, and Multiplayer (with player to player collisions of course)
I would love to be able to use the modified Buffalo MPCV used in Transformers (2007): Would be neat to fling around cars with it: There's already a mod of it for GTA SA, so it shouldn't be that hard. I might take a stab at it after the Mariner's finished.