They should just make an end game quest.
"Audience with the Dungeon Master".
Very specific requirements to even enter the 12 man raid. Must have all favors. Must be completionist. Entering quest resets all favors to ZERO.
Challenging puzzles that need at least 6 players to operate at the same time.
Fights with former raid bosses but they are all buffed up.
Random traps that can one shot you.
No shrines except to the door of the Dungeon Master.
A Dungeon Master so powerful that if the quest is completed, it's likely only one or two players were left standing.
The reward..for those that died, a named item of your choice. The one's standing, a named item and one wall of the most sought after Trophy Room.
The wall will be daggers, next wall will be shortsword, etc. Once an item is placed on the wall, it can never be removed.
That should keep the very best busy for awhile.