You could drop a special key for all players who reached endgame.
This special key only drops once in a month!
The key allows the player to enter a "special" dungeon/or instance
The player can take some friends with him (maybe a group of 5?)
The player can also choose to sell the key in the auction house or directly
The weapons, costume pieces, additional content (maybe auras or something like that) you find in this special instance can only be found in this rare dungeon. Not in the shop, not elsewhere...
PROs:
Everyone will be exited! "Who will find the special key this month?" It motivates players to log in every day. Maybe a friend found it and needs a team to go through this special secret instance?
The loot is rare and can´t be bought. So there will be paid high prices in auction houses. Super expensive objects motivate players to make money.
Maybe you could drop it for everyone? So if the player has not reached levelcap yet, he could sell it in the auctionhouse.
If a player finds the key and don´t use it, the key will disappear at the end of the month.
Oh and... of course there will be a superstrong opponent at the end...
Note:
I was trying to find some exiting endgame content, some fresh and new ideas.
sidenote:
You could use the same model for a weekly drop but with other loot and another dungeon of course.