One of those ideas that appealled to me was the use of old pantyhose to use up old soap. There's an article on Lifehacker, and many more if you do a search.
What's interesting about this idea, is that the consumers' modification is actually a significant improvement, not only does the method use up the old pieces of soap, but generates a better lather (due to a higher surface area to volume ratio) and has a built-in loofah.
So why haven't the soap companies cottoned on to this? At worst, it's a gimmick that could increase sales. At best, it's a differentiator: more lather; better for the environment (as no wasted ends) (demonstrating the companies' environmental credentials); works with cheap soap; less likely to slip out of your hands; does not slip off or fall through soap holder; exfoliates (the loofah element); and due to the better lathering, it uses the soap faster (which means more sales).
Another possibility would be to sell small soap beads specifically for a bag. The beads could be of different colour and odor, allowing to the customer to mix-and-match to get the smell they wanted.
The lesson of this? Companies need to pay attention not just to who's buying their products, how much they're buying, when they're buying, etc. But also, how the customers/consumers are using their products.