Why try to guess what many people would want? Let them tell you more directly via a service for collaborative startup specification. Unlike implementation, specification -- at least at an informal level -- can be done by pretty well anyone. Of course, you will need to provide some way to end up with a coherent spec from all those user contributions. But if something like wikipedia can produce stellar results, then why not this?
And when you start working on something, why not keep all those people up to date with your progress? They would probably be very eager to see what you are doing with their spec contributions. And they may evolve the spec over time, again, in a collaborative way.
When your startup is ready to launch, you will then have all those people ready to try it out and tell their friends.