If it does happen, it's going to be a long wait
Speaking to IGN about the possibility of Forza Motorsport 5 getting night racing or weather effects, creative director Dan Greenawalt emphasized the amount of work that would need to go into such additions to the game. "When we do things, we do it all the way," he said. "That means physics, that means changing conditions, that means everything. So that is not the sort of thing that is easily undertaken in a patch."
Continuing, he explains that "To build a new track that would work at night and in wet would mean a bespoke track ... If we wanted to make one of those tracks work with the added particles or projected shadows, and of course adding the physics to do something like night and wet, it means re-engaging those tracks. I’m not trying to give an excuse, I’m trying to give context as to why this is an order of magnitude higher than something like Drag and Tag, and other things we're looking at from the community."
The creative director did note "it's fair that people ask for the features they want, and we're here to make fans happy," but, don't count on this stuff winding up in Forza 5. "It's not where we chose to put out investment," he said. "We chose to be 1080p and 60 frames per-second, and have that solid performance."