If you do a Kickstarter for it (which I VERY MUCH hope you do), it'll go from "impossible to turn down" to "you may end up with a mildly creepy nerd stalker munching Doritos and staring at you in the dark as you sleep."
I am thinking of using The Lost City of Barakus, a sandbox-style module from Necromancer Games explicitly designed for levels 1 - 5. It seems like a perfect fit for the Beginner Box! It includes a starting city (along with some city adventures), some wilderness encounters, and a medium sized dungeon. It's out of print physically, but available in PDF on RPGNow (and physically on the secondary market).
If I could cast Wish or Miracle, I would like to see this product as a springboard for an AP set in an earlier (more "mythic") time period of Golarion. Come to think of it, this could also be a great way to satisfy the itch of those who want epic/mythic adventures. Put it far back enough that it wouldn't interfere w/ modern continuity and/or hesitation at having high-level material in today's Golarion.
Or just a "regular" AP (starting at level 1), but set in the mythic past in one of these great kingdoms of old...that would be epic in and of itself.
Agreed--that would be a nifty 'setting within a setting'.