If you've done a market study, then you know who your target market/audience is. It is essential for you to get some end-users in that market to give you some market information on what type of product they would like to see. Find out what the user's PAIN is in this market, especially in terms of what your product can do to alleviate this pain.
Going boldly forth without testing (even if it is not a full prototype) is going to be a wild stab in the dark. Look at the prototype stage as an opportunity to save time, because you should be focusing on 1) What works, 2) What doesn't and how to fix it, and 3) what opportunities did you miss?
Along with this, you may find that the way you approach the market has an impact on your prototype as well, so how you access the market (i.e.: how do users become aware of the product, what influences their purchasing decision, and what is their budget or value proposition for the product) will also shape the final product significantly. So make sure when you talk to potential users in the prototype phase that you're capturing this stuff as well. The "Voice of the Customer" is one of the leading success factors that many often skip and then end up with something that misses the mark... by the time you've corrected and integrated that voice, you've given competitors a chance to adapt/adopt and you've also established a reputation.
