All 5 Mooshu guardians- Earth, Water, Metal, Fire and Wood, in nightmare mode. The leader would be a terrifying metal guardian- except this time he has black and red metal armor. The dungeon would have elements of each boss present. What I mean by this is like, there will be cages needed to activate metal guardian, earth guardian can summon terracotta minions, fire guardian can summon something as powerful as flames, etc etc etc...
Drops include complete set of gear for each guardian, e.g. obsidian metal guardians sword. Further unique drops include a new obsidian kirin mount (no idea how it would look like), a wood guardian pet or something, etc...
Out of all these suggestions, the nightmare guardians is my favorite, and also I think it would be currently appropriate to implement.
Nightmare Armada Elites
Probably not appropriate to implement at the moment, since Kane is already a level 70 dungeon. Perhaps this should be accessible to level 85 pirates in the future. In essence, its the current Kane boss fight, but much harder and with new cool armada drops. A few marines and dragoons here and there as well.
A Nefarious Nightmare
Yet again another fight that should be for level 85 pirates, maybe even level 95. Not only will Moo and 3-4 terracotta minions be present, so too will the nefarious five. Insanely hard fight, but doable. Drops include upgraded nightmare nefarious gear, upgraded nightmare imperial, a nefarious dragon mount etc.
Nightmare Spiral War
These enemies would be from all over the spiral. They are essentially bosses that are somewhat underappreciated. I suggest the following lineup: Buck Bronco, the combined Minotaur, general Tso, Fin Dorsal, Gortez and Guy Fox. Drops would be unique
Like the scratch dungeon, but with other companions gone nightmare. I suggest the following boss line up: El Toro, Bonnie Anne, Ratbeard, Catbeard, Monkey King and Contessa.
The Ultimate Nightmare:
Essentially one classless pirate boss. Looks like a shadow person. Insanely powerful. Can revive three times after dying. Rumor has it, it is the shadow version of the pirate itself.