This thread is yielding a pretty good collection of links and titles offering bits and pieces for the aspiring sandbox builder - but frankly, I have yet to see a campaign setting / mega-module that truly, fully delivers on the concept. What would that look like?
My ideal product would contain ...
- a high level setting overview, broken out into player's guide and DM section
- a rules/mechanics-revamp to offer more interesting choices wrt outdoor adventuring (movement, exploration, sustenance etc)
- a poster-size, detailed map at 1 to 5 miles per hex, densely seeded with special locations
- detailed descriptive for every location on that map, for actual use during play, including an atmospheric read-out, plus background info for the GM, much ike a dungeon-room entry
- extensive regional random encounter tables (day/night), including noncombat encounters
- a "Book of Lairs" style collection of embellished encounters, including small maps
- a collection of small to mid-size static adventure locations (dungeon magazine calibre)
- a dozen or so interconnected, module-sized adventure locations of various levels
- a few small but detailed towns, complete with map, shops/services, key NPCs
- all tied together by a dynamic matrix plot with NPC villains and heroes that the players can get involved with in many ways, and to many effects.
... and then the drugs wore off.

But seriously - isn't it baffling that after 30 years of roleplaying game design, the number of supplements/modules that actually offer the open, dynamic, player-driven plots that are made possible by the format can be counted on one hand?
Question then:
Why is that?
My suspicion is that they are actually very hard to think up, organise and playtest, but I'm curious what you think about this.