What's the least we could do, that would have all the necessary functionality to be shipped. In software/web industry you can change your product every day. Don't make the developing list be set for the next year or two.
Rest assure, the requirements for what's needed will change as you go along. Figure out what the necessary features are, and filter out the nice-to-have's, build quicker and deploy.

Text: Smidig 2008 [no] Picture: Modern Art