Rather than making heroic and nightmare mode harder just by mob hp, damage and enrage timer why not add extra objectives to the instance or other aspects to the boss fight or even adding optional bosses.

for example

normal mode (bosses as usal)

heroic (bosses + extra aspect be it extra adds, a bomb to defuse that gets re armed threw out the fight, weak spots on bosses that become vunerable/imunitys or shields that need to be broken to carry on)

nightmare mode ( add enrage timers or even a global area timer ie the false emp if you dont kill malgus with in 30 mins the ships self destruct is activated)

there are also other issues with the current pve tier system for one the loot rewards doing eternity volt on nightmare mode as far as i understand gives the same loot as heroic mode

why even bother doing nightmare mode if this is the case?

solution rebalance the loot give higher mods for example in nightmare mode, add a title for each hard mode

if you go with my 1st idea where nightmare mode has more aspects to a fight than normal why not add in extra cut scenes to be viewed just from completing it

dont know what you guys think but at present i feel pve doesnt really reward you for "hard mode" lol

in other games you would be treated when you completed it on the hardest mode even if it was a 20 second movie

another option is to add bosses that are only doable with in a time frame so that you have to kill certain bosses or take harder routes to get the "nightmare bosses"