I assume you all know what the Nightmare mode is, so i shouldnt have to explain that.
Anyway, there is a few ideas i have (realize that these are ideas towards the wiki community, i know DE dosn't come here for suggestions) about how to impliment some Nightmare modes into actual gameplay.
Low Gravity: when breaking corpus ship windows, there is a knockback effect followed by what is supposed to be the sound of air being voilently sucked out of the area. To anyone that ever says "wouldn't that make it impossible to breathe? i mean all we lose is a little health over time", there is still air in the room, what is reducing is pressure, so realistically speaking your frames still can breath (at least until you die, obviously) but the pressure is slowly venting out into space. the point is, during this time you would think that the loss of pressure and/or the failure of localized artificial gravity would decrease over time. it would be nice if the gravity level decreased along with health, and adding a slight tug against players towards the broken window, as being sucked out, and apon being thrown out into space you teleport back inside. (or on nightmare missions you die)
Vampirism: This one is a little tricky to properly impliment into the game but you could in this way, each member of the team has a special artifact placed on them (like on the chest or something) that slowly converts the user's life, into energy. eventually the player will die but when killing enemies it will glow slightly, adding health. this mode would only be available to level 30 warframes as it would be a special task by the lotus herself, essentially for the elite players.
No shields: this could be implemented on normal missions when entering the outside worlds of some levels, as if the atmosphere is ionized (like corpus levels, for their protection) and keeps shields from regenerating (but you still start with your shield). this would be on tier 2 missions and only on a rare chance.
High Level Enemies: this is already implimented in game, with higher level enemies like elite lancers being introduced more and more the higher level you go, but this would include the lotus exclaming that an elite force has been called into the level and to take extra caution.
Longer Reload: tricky to impliment into normal missions, but it could go like this. On infested levels, the spores in the air have intruded into your weapons mechanics, making reloading more difficult, thus slowing the reload speed, starting at 10% and going up with level. this would only be on special missions like alerts that have alot of enemies in them.
even though this is just a quick thought that will never be implimented, it would be nice to hear your ideas on these :D