Let's go with some basic requirements for now:
  1. We'll be making a new difficulty tier for an existing dungeon.

    We'll focus just on the dungeon's mechanics, gameplay, and tuning of mobs and bosses only relative to the weakest/strongest mob/boss; pretend the ilvl requirements you may prefer don't much affect the experience (e.g., perhaps the difficulty and its rewards scale with your ilvl), unless you specifically want the dungeon to be easily cheesed as you progress, and that the dungeons grant sufficient rewards.

  2. You are free to adjust the pathing of the dungeon through its environment, up to the extent that a Hard Mode dungeon (renamed to "Return to...") would (i.e., quite a lot). You can also add forced path-randomizing elements if you like.

  3. Bosses should be decently difficult, but in a slightly different fashion from Savage Raid bosses (e.g., typically with a slightly less stringent or esoteric learning requirement than seen in Savage, focusing instead on more general coordination and tactics).

  4. You must make trash at least moderately interesting. This can be through indirect challenges, or synergetic abilities of the pack itself. Dealing with any decently difficult pack should feel strategic.

  5. You are free to give certain jobs indirect advantages (by way of their kits) for certain encounters or even direct advantages in pathmaking / RP-ish events, etc., but are expected to try to spread out the love if so. (You don't need to lay out every advantage; just as least note to what extent you would like to see jobs feature in the dungeon.)

  6. If useful to the above, you are free to add a further Challenge requirement, be that through the likes of clear speed, having mob spawns increase in density or power over time, minimizing party damage taken, etc., and may add compensatory buffing mechanics synergetic to such. You can have these be obligatory or optional, cumulative or mutually exclusive, whatever you wish.



P.S.
If you really, really have a pressing idea for a new dungeon, feel free to go with that instead; I'm suggesting revamps of existing dungeons only because then we all have the view in mind and can copy and then just adjust the existing maps, rather than having to make anything wholly from scratch.