Here's a simple formula for an epic sandbox campaign. Start out with zero experience characters and play through an intro module or two, Keep on the Borderlands or something like. Add to the treasure vault of one of the most badass critters in the module the first section of the Rod of Seven Parts (stats for which can be found in Eldritch Wizardry, but I like the associated fluff in the 1st edition DMG better). We're going to cheat and only slip the Rod into the loot when the PCs find it.
Once the PCs have the Rod in their possession have an Elminster type show up and give them the bad news: they are the Chosen Ones. It is their job to reassemble the Rod to prevent the return of the Wind Dukes of Aaqa. The Wind Dukes created the Rod and only one of their own devices can be used to thwart their return. Most of the ancient sources depict these guys as angelic friendlies who used the Rod to save the world from the forces of Chaos. When they return they are to usher in a new golden age of purity and light.
They did turn back the Chaos, but in human terms these Masters of Law are just as dickish as the Abyssal Hordes they fought. To the Wind Dukes the idea of a perfect Prime Material Plane does not include such a corrupt concept as "matter". That foretold "purity and light" stuff was a literal description of their goal. They plan to burn the whole damn universe down to its subatomic components in the name of winning the Cosmic War on Chaos.
As the GM, all you need is a sandbox to drop the rest of the pieces of the Rod into. The rest of the campaign practically writes itself. If you've got Points of Light and Points of Light II (which you should totally own if you don't), just place each of five segments of the Rod in Wildland, Southland, Borderland, the Golden Shores, Amacui, and/or the Misty Isles. Put the last piece in Acheron or that awesome hell-volcano in the second volume. You could even roll dice for placement. If the Rod segment lands in an otherwise empty hex, build a dungeon around it. The net is lousy with free dungeons that could be pressed into service for such uses.
Or you can use the Wilderlands of High Fantasy for this campaign, if you've got a copy. Just roll a d20 for each Rod segment to find out what map to put it on. Map 19 can be found in Fight On! #3. If you roll a 20 I suggest placing the Rod segment somewhere in the City-State of the Invincible Overlord. If you're crazy, use a Traveller sandbox like the Spinward Marches or the whole dang setting and run the campaign in the Encounter Critical style, using EC or Rifts or something as your system.
For added fun, build a couple of rival NPC parties trying to get the Rod away from the PCs, one of Lawful-type Wind Duke sympathizers and one party serving Chaos. If a decent parlay can be arranged either group might be convinced to help the party, since the Wind Dukians probably don't realize how screwed they are if their side wins and the Chaos side's primary goal is the same as the PCs.
Once the PCs have the whole Rod, they have to rush to some apparently random point on the map where they can seal the portal between the Wind Dukes' realm and the Prime Material. If the PCs drag their feet, set a few minor Dukes (i.e. Godzilla-level threats) loose to stomp around the campaign world like the Vorlons in the fourth season of Babylon 5. They will, of course, attempt to burn down all the nice places the PCs have visited in their travels.