Damage would be awesome, but I just have to go with day/night cycles and variable weather.
Variable weather has long been overdue. In GT1-GT4, tracks were too static... each track just had one absolute condition, and frankly, it was really annoying. I want to be able to drive on any track under any weather condition (within reason... no tornadoes!).
And day/night is simply needed. Really killed the realism of the endurance races of past GT games. Plus, I'd like to be able to race on tracks at whatever time I please. Tracks of past games were just always day or always night... kinda lame.