I have a one-shot/tournament Monster of the Week scenario that would be rather easily reworked into this game.
consider something like the Tournament of the Dragon from Champions 3rd or 4th edition….every 60 years it’s held to choose the most worthy warrior to face and defeat the death dragon in combat…the winner gets a few mystical boons as well (assuming the Death Dragon does not defeat them, is not released and does not destroy the world), or Mortal Kombat (lose 10 Mortal Kombats and get invaded), or Fate/Stay-Night’s Grail War.
Every 60 years, five artifacts are created and distributed as part of a re-occuring ritual. People seek out and earn these relics and then try to win the remaining relics from the others, finally using the five to reseal a big bad evil thing (I usually go with Typhon from Greek Myth). Only the boons come from the magic siphoned off the Big Bad Evil thing, so some of those immortals that have participated multiple times start to turn evil…and the cult that initially created the ritual to gather power has turned to evil and seeking to release the BBE…
The one-shot is assuming every artifact has been collected from around the world and the various parties are meeting to have the confrontation and decide who gets to perform the ritual. With the players being the group holding one of the artifacts.
The campaign would be first spent learning about and getting one artifact…followed by learning there are five of the items and then heading to the city of the ritual in order to compete in the tournament.
It could easily be scaled from a global competition to a city one.