An addition to the good answers we have already: If you have a large gaming group, with a reasonable degree of consistency in play style, moving back and forth between different DM's worlds can become a routine activity for characters.
I met this at college in fall 1979, where there were people in the group who'd been playing since 1974. The game mechanic was an institution called "The Halls of Teleportation" which existed in various towns in the different game worlds. Transport was free for adventurers, since, on the whole, they brought economic and social benefits to the worlds. Merchants could ship stuff between worlds, but had to pay a percentage. The towns continued to operate their own legal systems, and would sometimes extradite criminals, but this depended on the crime, and the worlds involved.
This came with some implicit lampshading, and an unspoken agreement not to spoil something that was of benefit to everyone by exploiting it too hard.
The result was a very large, multi-DM campaign, that was far more varied and interesting than any one DM could produce. This emerged naturally from the initial conditions, and still exists, although it isn't played as much now as it was in the late seventies and early eighties. Then, most members of the group spent the whole of each weekend playing or DMing. You can build a lot of gaming experience in a few years that way, and we did.
Of course, people had more than one character. Most of us had tens of characters, and would start new ones regularly at first level so that we always had a variety of characters at a variety of levels, and could join any expedition that sounded interesting and had space. An average party might be 4 players with a total of 8-9 characters, but this varied widely: some people didn't like playing more than one character at a time, while others were happy with it.
There would be ongoing plotlines, and reoccurring villains. There would usually be a core group of characters interested in a particular plot, who would work on information-gathering with the relevant DM, and recruit more characters for an expedition or attack when they had a decent plan. This meant that many scenarios were organised by the players, which definitely eases the DM's workload.
... it's viral!
The metacampaign I know spread itself over several British universities in the late seventies. I helped spread it to another in the mid-eighties, simply because there was another one running there which was pretty compatible. That one doesn't have Halls of Teleportation, but requires the characters to have ways to travel between worlds, after which it's then easy to move between the various game worlds.
I'll be running a scenario that's definitely part of the metacampaign at a convention in January 2018, 38 years after I met it. I look forward to playing it more when I retire.