The customer's voice
It is likely that your next big product idea may be found among your customers! The problem is that the customer would not be aware of that if you asked them. In order for you both to discover that idea, you need to do things together. One way is to arrange with your customer to spend an agreed period of time at the workplace with them, when you think they are most likely to have ideas about their job. This is particularly appropriate when dealing with products that are used in a way that you and your company don't really understand or know how to improve.
For example, a company supplying machinery for bakeries was looking for ways to improve its products. After two weeks on site with a baker they came up with variety of ideas for new products that they had never thought of before.
For fast-moving products, working in, for example, the shop, the bar or the restaurant can give you similar experiences to bring back to your own product development.