I don't know the answer to this question, but your customers do! Do you ever ask them?
Laurie Clemans wrote recently about passing a country store with a sign that said, Wave if you can't stop in. Unable to resist, she made a quick stop and went in to look around. When she failed to buy anything, the owner asked, "What should I stock so you will have what you need the next time you visit?"
Now think about that -- asking customers what they want! You don't have to be a retailer to ask questions of your customers, although admittedly it's easiest to do when you're face to face. But how many retailers bother? And how many non-retail businesses make the extra effort to find out what customers want.
Next time you make a sale to a customer, ask what other products or services they would be interested in. In fact, ask all of your customers. It won't be in your interest to offer every item that every customer wants, but if enough of them express an interest in a category of product or service, you might have uncovered a new opportunity.