The first campaign that I ran in my fantasy setting Jehannum¹ dealt with epic deeds, and ended up with the PCs radically changing the political situation. When it petered out I replaced it was another campaign for unrelated characters set a hundred years later. After that, I ran another champaign set two hundred years after the first in an alternative future in which the PCs of the first campaign had done none of their exploits of the campaign, i.e. in the future that would have existed if the first campaign had not happened. Then I did another campaign set two hundred years before the first, which produced a history incompatible with ending up at the political situation at the beginning of the first campaign. I have also done a campaign set eight years before the first (which prevented the war-of-succession that led to the situation of the first campaign), and no fewer than four campaigns set two hundred years after the first, which involved the PCs participating important political stuff and that were quite incompatible with each other.
Gehennum has had at least seven alternative histories in various campaigns.
@whswhs I know has set a campaign in an alternative history of Tolkien’s Middle-earth.
¹ Sometimes “Gehennum”.
Only in the sense that once PCs start hitting it it tends to go alternate fairly rapidly…
Though it appears that in my WWII-with-magic game there have been at least two timelines, one of which isn’t accessible any more.
I’ve only once used a setting twice: I’ve run two Transhuman Space campaigns. I don’t think it was possible to tell whether they were the same timeline or not from anything in play; I never decided that they were different timelines, though they could have been.
I’ve run two campaigns that were intentionally alternative campaigns to the original source. Agemegos has referred to Under the Shadow, where I put the divergence point at Sam not catching up with Frodo as he crossed the Anduin, but instead being captured by Saruman’s orcs. The other was DC Realtime, which I co-GMed. It started when I observed to the other GM that it would be interesting if DC had started out their heroes’ careers when they were first published, and aged them realistically after that. He liked the idea, and we worked out the theory that the Crisis on Infinite Earths had produced a single collapsed timeline where that had happened. So by the time the campaign started, Batman had grown old and died, Superman had some distinguished gray at the temples and was spending his time on applied physics more than fighting crime, and Wonder Woman was still agelessly beautiful—and Captain Marvel was still a young man, but his alter ego was a retired college professor with a weak heart.
I didn’t design my Barrayaran campaign as an alternative timeline, but it became one; for one thing, I had Mark Vorkosigan married to Kareen Koudelka, with two daughters. That’s what happens when the author writes more fiction of her own after the campaign starts!