Concept: a crossover-focused RP setting which is centered on a specific location/region/city or whatever, where a lengthy series of disasters is stated to have taken place in the past -- weird magical storms or whatever, attacks by hordes of monsters of all descriptions, and so on. This all went on for an arbitrary length of time, and ended an unspecified number of years ago; maybe that kind of stuff is still happening, and that's where the RP begins!
Like I'm thinking of some kind of RP MUD, or other venue where dozens of an arbitrary selection of players drops in or out of specific scenes, rather than a focused tabletop RPG with a small, fixed group where you can set up the RP as you like; this is all set up specifically so that any character who had some sort of "Huge Backstory-Defining Incident x number of years ago" in their canon can insert themselves into the plot without having to retcon anything new into the setting's existing backstory ("Remember the demon attack ten years ago?" "No.").