The answer: the Open Gaming Table. Set up a situation where you don't need the same players every week, just play with whoever happens to be on hand. I'll quote the Alexandrian's basic OGT structure:
1. There’s a huge dungeon. So big that it can’t be cleared out in one or two or even a dozen gaming sessions. In fact, it’s so huge that the parts you’ve already cleared out will probably start repopulating with new monsters before you finish exploring the rest of it.
2. There’s a nearby “gold rush” town where PCs can form adventuring parties to explore the megadungeon.
3. At the end of each session, everybody heads back to town. At the start of the next session, a new adventuring party forms and heads back to the dungeon.
He goes into a fair amount of detail on how he sets it up for his OGT. As a comparison, I'll go into some detail on how I set mine up.
First of all, it's set in the same world as my first campaign. But, the deiteration of one of the six iterations of the machine reduced the machine's total power to 5/6.
For my first campaign, I had calculated exactly how much the seas had risen by. I ruled the world had been basically earth-like in its geography, and the ocean had risen to about half a mile above the top of the highest mountain. Everest is about 5.5 miles high, so the sea rose by 6 miles. How convenient that the math worked out! 5/6 of 6 is 5, so the sea fell by a mile after my first campaign.
I didn't bother to calculate how much exposed land there would be if the sea were 5 miles higher than it is. It turns out there are 14 mountains more than 5 miles tall, so the world should have been something like 14 smallish islands. So I departed from reality and set up 5 mini-continents. The elves settled one and called it Miranda, the engineers settled one and called it Surface Holdings, the humans settled one and called it Romus. One is still unsettled, nominally controlled by the (aquatic, non-amphibious) sahuagin, and is called Here There Be Monsters.
The player who played the scout in my first campaign is a massive
The vampire who defeated the adventurers in my first campaign was lavishly rewarded by Quasxthe. He was a minor baron in the Disreputable City, and Quasxthe displaced the rest of the family and made him Count Rogan, nominally in charge of the city. But it turned out that the orb had been what was keeping the city floating in the first place, so Count Rogan also asked Quasxthe to land the Disreputable City on a mountain (actually on the merfolk city of Mat~sya, but nobody cares about the merfolk, they would need to abandon the city anyway).
Rogan claimed the continent, named himself King, and called his nation Gus, after Gus Dreadworm, founder of the Disreputable City. The people of the Disreputable City moved downhill to a new city, which they called New Disreputable City. That name crosses the line of unwieldiness, so they just started calling it NDC, which was very quickly corrupted to Endeesy. The city of Endeesy is structural element (2), above, the "gold rush town".
The king was also rewarded with a pirate captain for his wife, the captain of the vessel the PCs had sailed on in the first campaign. Later on, the king was assassinated in a manner suspiciously similar to some PC-on-PC shenanigans the wizard pulled in the first campaign, leaving his half-vampire son in charge.
Then, as the waters receded, it was discovered that Gus and the Omorashi Empire had claimed the same continent. The entire military and diplomatic corps of Gus has been occupied ever since, dealing with the border dispute, leaving domestic issues ignored.
So the new king and his court is of the opinion that there's no problem that can't be solved by posting a bounty with the Adventurer's Guild and letting adventurers team up to handle it. This is structural element (3), above, the group that doesn't need to be the same from week to week.
Without too many spoilers (in case my players are reading), I'll go into more detail on the dungeons and the bounties in a subsequent post.