[samdk]

Talk to people.

Don't ask them what they want you to build--they probably don't really know. Ask them about the problems they're having, and work with them to solve one.

If you've done a good job of that you'll have at least one happy user, and they probably know others with the same or similar problems.

[alexro]

People have all sorts of problems, so restricting your efforts to a domain you care about is probably the most efficient approach