My ideal one would be:
None of that designer costume stuff. I don't want to buy clothes, and the shops just clutter up my map.
I'd say no bikes, but I believe in Test Drive Unlimited. And my ideal game with no bikes would contradict that.
No paying for damage, or no damage in the first place. TDU is not about crashing and damaging cars, its why the first game is so brilliant.
Lifelike performance I'm OK with, but what's wrong with just having some fun in a McLaren at 425kph? there are more serious driving games out there. TDU is a bit of a laugh right now, which is a good thing.
More cars - Fast ones, slow ones, minivans, SUVs, trucks. Everything you can get. I want lots of cars so I never get bored.
No day/night
No pedestrians (sorry, that'll just make the game Grand Theft Auto)
Weather, or actually just rain. And sun.
Some offroading would be good
Improved graphics
Some more racetracks, or the ability to actually race on the Oahu raceway. I dunno what you can do in multiplayer, but in single player the raceway is a pointless strip of tarmac with no challenges on it.
I don't really car about any of that though, since I'm happy with TDU as it is (gameplay), all I really really want is more cars.