Oh yeah, night racing. Has been suggested quite some times now
I'm not too sure about its usefulness, though, considering the races are already messy enough as is. IMO this would be a feature that is cool for 5 minutes and then turned off as you realize driving in a black void doesn't necessarily make racing more fun.
A day/night change involves holding multiple textures on the graphics card which could isolate users on lower specification systems, either a very slow incremental change could be done with texture loading on an IRQ - or the minimum requirements would need to go up to store the extra textures.
One thing for the devs to bear in mind is that different parts of the world experience a different speed of day/night transition, which only serves to make it even more complex...