KitKat are already full of wafer so Mondelez wouldn't be motivated to 'put anything in the chocolate bar at all so long as it's not chocolate'. What's cheaper than wafer? I guess if they can make more of the inside wafer, so there's less chocolate coating?

Perhaps you can make wafer bubbly, so it's more air? Although it's cheaper to just put it in a plastic wrapper and fill the wrapper with nitrogen...