Everything I really want already seems to be in the game. I'm far from hardcore so I won;t embara myself attempting to talk about tyre width, traction and the like.
What I want, more than anything is a proper garage. A parking lot of sorts, with 100 spots, one for each car you can unlock. By default, the lot would be seperated by manufacturers, Ferrari's in one section, Subarus in another et cetera. However, you could switch cars around if you wish, so you could have all your supercars in one section, perhaps a filter to automatically organise this for you. (1000 is a lot of cars, obviously)
Then, imagine you could customise the garage itself. Type of garage (bunker, parking lot, indoor, outdoor,) lighting (daytime, streetlamps, flourescants etc,) time of day.
Then when you've got your perfect garage, imagine you could free-roam and take it into Photo Mode. All the cars a man could ever need, in a glorious setting, with the colours of your choice. If there's space, you could perhaps have 2+ of the same car, maybe with different colours to stop the place looking too empty.
As well as looking awesome, it'd give you another reason to flesh out your collection as much as possible, the feeling of filling your garage with every car in the game would be fantastic.
The obvious problem of course is one of processing power. I'm sure that Kaz and co. have thought about it, asits hardly an original idea, but I'm not sure that the PS3, powerful as it is, could handle 1000 exceptionally detailed cars on screen at once without constant load screens.
I prefer to go into games cold, so there may well be something similar already in the game. If there is, I'm sorry for wasting everybody's time...