|Dale McCoy Jr President, Jon Brazer Enterprises|
Something on the scale of Paizo's APs or the 64-page modules, you really do have to prep plot if you want any and railroad the PCs to some extent. That's what you're selling. A prepared adventure. If you want to have prewritten stuff for the PCs to encounter, you've basically got to decide where they're going to go and roughly when.
I tried the plotless adventure once with Encounters and Maps: Cave of Kobolds. Emphasis on the word "Once." I had a whole line planned (2 more of them are all but done and have never been released), but the review entitled "Avoid" with the line describing it as "some thing I could knock up my self in Herolab given a few minutes," killed that idea for me. Sales were beyond terrible, and it shattered any confidence I had in myself for even trying it. It was years before I tried a proper adventure.
It taught me that people pay for plot more than anything else.