I've played a lot of racing games over the years and seen a lot of effects overdone. Don't implement god-rays and fireworks, this isn't Grid2. No offence meant, NightCreature83 ;-)
Here's a few things I think you'll need: motion blur; environmental reflections on cars; shadowing on the track and cars from nearby objects; shadowing on the track from cars; probably some kind of tessellation for curved road and scenery sections; smoke; dynamic lighting from headlights/brake lights; very nice shading models for the track and cars.
I can't stress the last point enough. If you're making a racing game you're expecting me to spend a lot of time looking at your simulation of some tarmac. I also probably drive a car myself in real life, and I instinctively know what a real road and real cars looks like. Your textures, shading models and illumination tricks need to hold up to that standard. Take into account new/old surfaces with different material properties, patches of grease or oil, tyre rubber impregnating the asphalt in the braking and traction zones on the racing line, wet surfaces and standing water etc. And some of these need to be dynamic, i.e. cars laying down rubber as the race progresses.
I know this is a lot, and I know that even the latest racing games don't get it right, but this stuff is the target. But start small, concentrate on maybe two or three of these effects that you think are most important. I'd say get the curved surface tessellation stuff going as best you can (polygon kerbs are awful) and then a nice material model for the track that just gets the fundamental light response right for night racing. Go for a drive, take some photos.
Don't worry about foliage simulation for now. If I'm close enough to see how your trees are rendered then I've already crashed. ;-)