[beat]

It's not free, and it's not simple, but Aha (https://www.aha.io) can be very helpful in figuring out product.

Engaging end users is a problem from a couple of directions. First, the very idea of "end user" - you need to engage customers, not end users. (Take Facebook, for example... you're not the customer, you're the product. If you build something for Facebook, your customer is Facebook, not Facebook's users.) This situation isn't exactly unusual. In this case, it's the customer's responsibility to gather feedback from end users - and put you in the feedback loop, as needed.