I really don't want to write a novel but I will try to explain the best I can.
It would be awesome to have new modes, new factions, new maps, but they all require new assets and fall into new DLC territory.
When you are adding a new difficulty, you are just adding one more permutation of enemy numbers to the existing ones, i.e. on the existing maps, instead of routine, threatening, perilous, you would now have routine, threatening, perilous, [super perilous]. This is a lot more easier to implement than asking for NEW things (which I love and want, please give us new DLC soon Bioware
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It's a lot of work to program new AI behaviour; normally, to make things harder in higher difficulty settings, enemies are just given higher stats and come in greater numbers. This happens all the time in RPGs. You go to a new harder area, you get to fight recoloured enemies that behave the same but have beefed up health and attack.
Of course, RPGS have level caps so that its easier to balance. There is no "level cap" to DAIMP promotions. I believe Wavebend pointed out that it only takes 100+ promotions per stat to make perilous the equivalent of threatening to a player with little to no promotions.
To up the difficulty, so that even players with 100+ promotions will find it challenging, enemies need to have a lot more health and damage. This would mean you will have to be high promoted to enter the new difficulty. For those who aren't, you will be tickling enemies with pea shooters while you die in one shot to their attacks
And in time, as people promote higher, the difficulty is once again lost as people approach 200+
I personally think there's a better way to add challenge. Instead of scaling enemy stats to the players in the new difficulty, scale the players' stats to the enemies by removing the effects of promotions. Benefits are:
- This difficulty is always relevant. It will always be hard, it never gets easier, no matter how many times a player promotes.
- This difficulty is fair. you don't need to promote a huge amount to play, but veterans will still have an advantage with their end game gear and their familiarity with the game mechanics.
- There have been several threads debating on how promotions negate the necessity of skill. This difficulty would take the promotions out of the equation and allow these people to compare their "strate skils" 
Nothing else changes. you can still promote to your hearts' content, and the benefits of your promotions still work on the other difficulties. You should still get experience and gold in an amount befitting a new higher difficulty. For people who ask why bother still promoting: from what I understand, people who play the much vaunted ME3MP are incentivized to increase their prestige and it doesn't affect their stats, but the game is still hugely popular.
This suggestion isn't a complaint on promotions. I'm not trying to gimp people with high promotions or trying to break onto the leaderboards or whatever other things that some people think I'm trying to do. I just want the new difficulty to be challenging but still accessible to the majority of the players.
TLDR - Removing the effects of promotions for the new difficulty isn't as wrong as you think 