How about "Master Trials"? Reminscent of FFXI's Salvage and Nyzul Isle that had 'Pathos" where players received one or several penalties and restrictions. They would be Expert Trials and High Level Dungeons, but with random penalties and greater rewards.
Penalties could be one or more of the following, randomly generated at the start of the trial. They could be permanent for the run (more fun) or removable gradually as you progress(similar to Salvage), though that's less of a challenge and less fun :(
Examples of potential penalties:
Attributes Down (All attribute stats -15%).
Skill Speed -15%
Enemy attack speed +25%
Healing Received -15%
Damage Dealt -25%
Armor Slot locked (An equipment slot other than weapon or offhand is chosen at random, and unequiped, and cannot be re-equipped until after the dungeon)
Accessory Slot locked (Same as above, but for right side equipment other than offhand or Soulstone)
Soulstone Locked (Must play as your base class e.g. Lancer, Maurader etc)
Damage Received +25%
HP-15%
MP-15%
etc...
The penalties could be global (everyone gets the same penalty), class based (only healers could get the -MP one, only tanks could get the +damage taken one), or each player can get a completely random penalty(any penalty on anyone, for true randomness).
For winning, the reward could be:
Double the gil.
Tomes x 1.25
+1 slot to treasure pool (dungeon drops an extra item)
I think "Master Roulette' has an apt name, because if you're truly a master at your role, it should still be possible to do Shiva EX or Amdapor Keep Hard with Conjurers and a DPS without a Leg or Foot Armor, and a Gladiator with -15% HP. The penalties would make it fun I think. Maybe make it pre-form only, to foster high level, fine play. Also make it such that it can only be ATTEMPTED once, to prevent a group from abandoning and retrying in an attempt to re-roll the penalties until they get the least detrimental(if you attempt and abandon/leave without winning, your daily bonuses are forfeit).