I’ve always dreamed of running a massive, interlinked “Eternal Champion” game, where the players make versions of the same character in multiple eras, in multiple game systems, each running thier own contained adventure. You’d switch off the different games at dramatic points, and run a metaplot that spans them all.
A standard fantasy-horror game using D&D or Shadow of the Demon Lord or Dungeon World.
A 1920s investigative horror game using Call of Cthulhu or Trail of Cthulhu or Tremulus.
A modern-day conspiracy/investigation game using GURPS or Modern AGE or Shadow of the Century.
A futuristic space opera/horror game using Traveller or Star Trek Adventures or Uncharted Worlds.
So, maybe your PC would be a rogue in the fantasy game, a private investigator in the 1920s game, an FBI agent in the modern-day game, and a spy in the scifi game. All of the investigations are linked to the same time- and/or dimension-spanning cosmic threat, and events in one of the settings either parallel or directly tie to events in another setting. You could get really meta if there were ways to communicate to parallel versions of yourself across the dimensions
Such a game would be epic, cosmically weird, require a HUGE commitment from your players, and would be EXTREMELY difficult to pull off…
…but if you could pull it off, it would be GLOIRIOUS!