Something I’ve been thinking about a lot lately is what kind of new mechanics any hypothetical new enemies could have to further challenge players. At the very least, it’s a fun thought exercise.
I’m not theorizing about specific enemies here, since theree’s basically just two routes that can be taken there; you can take lore enemies and put their mechanics into the game, or take an enemy (possibly even generic) and give them an ability that is based in specific gameplay styles. For the former, we see Gutter Runners and Ratling Gunners, in the latter category we have Blightstormers and Leeches. I’m not saying either approach is right or wrong, both types are equally valid and can create interesting gameplay experiences. 
So, some ideas I’ve had:
-Temp Curse
Perhaps colored blue so as to make it differentiated from the red bars. The idea is that certain enemies might give you small amounts of curse upon hitting you, reducing your max health. Over time, this will decay, but still leave the health empty - it can be restored by healing or with temp health.
I feel like this might help counter the characters hit-trading on the idea that you can just generate more temp health. Of course, I wouldn’t think all enemies should do this, more like some kind of elite.
-Smoke Bombs/Something similar
The idea here is that some enemies could create a cloud of cover for themselves and other enemies around them. This will allow more enemies to close with you and not just get shot down, which would be particularly dangerous with disablers mixed into the group.
-Enemy Buffers
Basically, what if some enemies buffed their nearby allies with a damage resistance effect? Could be a visible aura, so you can know clearly what’s going on. Killing the “caster” would obviously end the effect.
The effect of this would be obvious, in that it’d force the players to seek the buffer and destroy them. While it’s easy to think “well they’ll just shoot him”, it still will put more stress on characters by adding another enemy on the list to consider about targeting first.
-Ult Disabers
Some kind of magical effect, that if hit or under the influence of, it would prevent you from using your ult and pause the recharge if it’s not up. This is kinda clear, as well, in that it will be a counter to clutch ult usage.
-Ethereal enemies
Just like it says, and pretty much what you’d expect. Some enemies would have an ethereal armor type that has a huge damage resistance, but low health. Regular weapons could still hurt them some, so you could kill them no matter what. Perhaps a new trait could be introduced that allows weapons to do better against them (and maybe more armor piercing?). Or, certain weapons with magical properties would be able to do more damage.
I imagine an ethereal enemy wouldn’t have a collision box for players, so you could actually pass through them. Maybe they’d give curse for when you do move through them.
Obviously I’d expect all of these things to have a skill-based component; I don’t think things like the ult block would be unavoidable, just more of a punishment for if you didn’t avoid it.
Any thoughts? Any other mechanics folks would like to see?