IMHO it would have to be sold as a boxed set. Boxed sets are more expensive to make, but you can sell them to chain toy stores - they look like your average boardgame and more people can find it accidentally. You can also include the dice - another draw for newbies. Plus a character sheet, a catalog for more, etc.
Keep the writing simple, but not simplistic. Don't talk down to the customer - kids hate this and they will hate the game if they aren't treated with respect. All rules should have a "For example" section demonstrating their use. Withouth those damned iconics, if possible.
A sample module is a must. This should be a mini-campaign versus something like The Sunless Citadel. That was good, but something more like Keep on the Borderlands would be cooler - maybe even something even more "dispersed". A small base of operations, several small adventuring areas (remember, beginning characters can't handle too much at first) and something bigger for "the grand finale". For example, you have the village of Mainbaas in a secluded valley, and places like "the goblin tower", "the ruined remains of Fallgor's summer palace", "the abandoned mines of rat-men" and finally the dreaded Temple of Terrible Terrors. Basically, the module must be non-stop fun for at least three sessions, including possibilities for combat, interaction with locals and so on.
The set should progress to at least level 5. The adventure game, AFAIK, doesn't allow character advancement. This is not good. Leveling is one of those alluring things in this game. Level 5 is good. You get the best spells of the D&D experience - Fireball, Lightning Bolt, Dispel Magic and Fly. Your character can take some punishment, but he will still not be invincible. Level 5 is where most good fantasy novels are set at.
No need to cram everything into the set. Prestige classes are a NO-NO. Magic items should be kept to about 20-30, limited to the basic types like healing potions, +2 swords and wands of lightning bolts. Include about 20-30 common monsters - some undead, some orc types, some detailed NPCs (guards and bandits and evil clerics) and a dragon or two.
And include pointers to the "advanced" game - this was done pretty well in the "Survival Guide" of the first PHB printing. The boxed set should be used to whet those apetites, all the while offering a glimpse into an even more fabulous world of adventure... That is, the core books.
Come on, there must be a d20 company willing to do this.
