There are a few game mechanics that really make the game inherently difficult to balance.
The "armor rating" system in this game is a flat damage reduction as opposed to the more conventional and logical percentage damage reduction. In example, a fully equipped person will usually have more armor rating than most enemies can hit for on routine, and a person with less than best-in-slot weapon will do less DPS based upon the relative difference and additionally the enemy armor rating.
The loot system in this game allows people to have weapons with ~5x more stated DPS but ~6x-one to two orders of magnitude more effective damage relative to the lowest damage weapons, and armor that provide damage reduction of anywhere from 25%-near infinity (1 damage per hit) relative to base armor depending upon the damage dealt by enemies.
The damage reduction from cunning, constitution, and willpower, become increasingly more powerful for each and every point into the attribute. To illustrate, if you have 79% ranged damage reduction and take 100 damage after applying armor rating, you will receive 21 damage, but if you have 80% ranged damage reduction, it is 20 damage, or a difference of 5% from a single point of cunning instead of just 1% if you were to compare 0% and 1% ranged damage reduction.
With such issues it is not possible for a difficulty to be designed to be challenging for people with BiS gear and high attributes without making the difficulty ridiculously difficult or impossible (aside from using cheesy or min-maxed builds) for people with low to medium-tier equipment and attributes. Perilous is almost trivialized for some players because of this, but do you really want bioware to make a new difficulty level while using the same inherently flawed game mechanics? I can imagine if they made such a difficulty, which I'm sure they are likely to, it will just be "who can be cheesier, the enemy or you?". That's what they should title it, don't call it Nightmare difficulty, call it Cheese difficulty, where people can see who has the most cheese.
If bioware didn't have so much damage discrepancy between weapons, didn't use their retarded armor rating system, and actually cared to balance abilities in-class and between-classes (especially ranged vs melee, there are very few valid reasons to not run 4 ranged classes if you are min-maxing), then it would be much easier to make different difficulty levels which isn't just about abusing overpowered abilities, using BiS equipment, and having ridiculously high attributes. But as things go at bioware, expect the cheese you are asking for.