Rain would be cool. Unlike bird poo, rain affects how you can drive.
Rain will force to use your brains to select correct setup and wheels, not to mention driving style. It's all about strategy.
Which is cool, when you got people with mouses doing record laps and then driving to a wall because they dont know really what racing is about. I get my kicks of it.
Also flat spots on tyres would be terrific! Play with mouse or keyboard, lock your wheels and reduce your chances of winning, yay for steering wheel and pedals!