If I where actually going to pusue something like this, this is what I would do:
1. Find a publishing company.
2. The adventure would be 50% combat, 45% role-playing, 5% puzzles.
3. 35% dungeon crawl (various locations), 35% city adventure, 10% wilderness, 20% planar
4. Generic, but familiar setting. (Almost Greyhawk)
5. Prelude and 6 chapters. Each different than the last to keep things from being stagnant.
Prelude handles levels 1-2. Maybe two different preludes depending on the average alignment of the party. After all, evil characters would likely be involved in the adventure for different reasons than good characters.
Every other chapter handles 3 levels each. Each chaper has different locations and goals with at least a month of downtime in between each.
6. Treasure and Experience distribution made to match the charts in the DM guide.
7. 3 books. Adventure Book, Stat book (NPCs complete with background and motivation and magical items, prestige classes, feats, and spells etc.)and a Player's handout book.
8. Cardboard mini's.
9. 1" scale poster maps of all important locations and at least 3 generic wilderness poster maps (maybe forest, mountain, and desert?)
10. Supplemental DM screen for random encounters, timeline, and other things specific to the adventure.
Such a thing would probably cost at least $65 or more, but it would be worth it to me.