The first image that comes to mind is a winter bivy bag with a good sleeping bag. Is that what you have in mind? Or are you looking for a one piece product?

Two quick thoughts. One, you want to make sure you have ventilation for foul winter camping to allow fresh oxygen in. Folks have sufficated in freezing weather when they got iced in with non-breathing material and no vents. Two, an outer sleeve that can be easily cleaned would be important, especially if there's no tent. Otherwise you'd have wash the bag more frequently and shorten it's lifespan by degrading the insulative properties faster.

Keep us posted. It's certainly interesting.