While we may still want Nightmare Mode for the Explosive Conflict Operation, it is still a ways off. To bide the time however, Lead Flashpoints and Operations Designer Jesse Sky took to the forum to add detail regarding the upcoming mode.
Quote from Allison Berryman
I asked Jesse Sky to tell us a little about how Nightmare Mode is going to differ from the existing Hard Mode for EC. He said that the numbers have been tuned up in every fight, and mechanics in every fight have been modified - in some cases, there are new mechanics that players will have to learn. The differences aren't extreme, but they present new challenges in every boss encounters, and the rewards are much more appropriate for the difficulty (unlike in previous Nightmare Modes).
For the thread in full, hit the official forum.