I'd add bonuses. These bonuses would be accessible via a series of jumps or a special route which would require skill to reach and would also add more time to your lap, if you fail to activate the bonus, you kinda lose out.
The route to bonuses could be idenfitied on the track via a coloured marker/arrow which would also indicate the type of bonus to be activated.
These bonuses might appear at fairly spaced intervals so races remain as fluid as possible. Bonuses I'd include......
The ability to activate traps for other racers. These would appear on the quicker / easier routes. So if you take the longer way round, you'll never see them. This would help to balance out the routes.
A way to slow down your boost red-line so you can boost for longer.
A way to speed up other racers boost red-line so it boosts for less time.
A way for you to access shortcuts to get ahead of the game. Shortcuts would stay open for a set time, so other racers close to you can follow.
e ability to 'Flood / Earthquake / Landslide / Fire / Fog / Hurricane' certain parts of the track.
Fauna & Flora
Animale / trees etc that inadvertantley fall / wonder across the track, causing you to wobble / crash, or causing temporary obstructions.
Speaks for itself really. A away to take out those big rigs.
Other Bits I'd Include
Add some weather effects to modify a track's behaviour. Wind could also be used to shift lighter vehicles through high jumps.
Being able to throw someone off/out of a vehicle. You take their vehicle, they restart on / in yours.