Hand each of your customers a few pieces of paper. Ask them to imagine that it is sometime in the future and that they've been using your product almost continuously between now and that future date (it could be a month, quarter, year, or, for strategic planning purposes, five years or even a decadepick a time frame that is appropriate for your research goals). Now, ask them to go even furtheran extra day, week, month. Ask your customer to write down, in as much detail as possible, exactly what your product will have done to make them happy (or successful or rich or safe or secure or smart; choose the set of adjectives that works best for your product).
The phrasing of the question is extremely important. You'll get different results if you ask "What should the system do?" instead of "What will the system have done?" (If you're skeptical, just try it.)