The idea is that you can preform a party and increase the difficulty and reward on any dungeon.

At the start of a dungeon, the party leader chooses "rules" for the dungeon in exchange for bonus tomestones on completion of the dungeon. The amount of tomes you obtain depends on the popularity of the choice (this could be how many times you have chosen it over a time period, or how popular it is on the server).

The restrictions and rewards should be significant enough to be worthwhile, but not make the mode mandatory. You can have as many rules as you want at once, but choose at least 3. Some rules I have in mind are:
-No AoE healing
-Trash mobs start running away in the direction of the boss room on low HP
-Trash mobs learn random skills
-(Most of the) Floor is lava
-Random divebombs and twisters
-Random 2.0 (lv50) raid boss replaces a boss in the dungeon, scaled down if the dungeon is not lvl60

Each rule should start at say, 30 tomes each and gradually go up if they are not popular choices.