I was just thinking about the Rankin-Bass holiday special "Rudolph's Shiny New Year" as a framework for a campaign setting.
No, wait, bear with me.
The cosmological set-up to the story is: every year at midnight on New Year's Eve, Father Time is replaced by Baby New Year, who embodies the coming year. The ex-Father Time retires to an island in the Archipelago of Last Years; each island is thus themed according to its year. The island of 1,000,000 BC is a mix of dinosaurs and cavemen, 4000 BC is Egyptian-themed, 1023 is medieval/fairy tale, and 1776 is American colonial.
If you drop the specifics, the archipelago actually makes a good basis for varied adventures. It doesn't even have to be islands; it could be multiple ruined keeps in a wilderness, each with a different theme. But you could use years as shorthand to determine a theme. You could even roll the year randomly: 4d10, read left to right, and either arbitrarily or randomly determine AD or BC. If you roll all zeros, roll 4d10 again and add four zeroes after the number rolled.