Player critical strikes deal no extra damage, but release a small staggering shockwave around each enemy hit (with an internal cooldown or weak stagger effect)
As players slay elites and specials, each gives 1 stack, at 100 stacks, a demonhost is summoned, upon killing the demonhost players are given increased attack speed and movement speed for a short time.
Give all trash units, non elite/special/boss more hp
When slain, all enemies leave behind a blood pool based on size, which heals or damages all units within.
Pox enemies fart toxic clouds occasionally or on death
Because I like the khorne modifiers from vermintide so much, Marked elites which empower units near them with bonus hp. Or gain hp from killing things, lose hp when not killing things
Friendly fire is enabled, and deals lethal damage for both allies and enemies
Force a monster at nearly every trigger
players deal x times more damage and have x% hp xdd
only one type of enemy per tier will spawn (horde, shooter, marauder, elite, special)
Every enemy gains stagger resistance
Every enemy loses stagger resistance, but moves much faster
Specials have much lower hp but deal more damage or have a much higher max count
All enemies take 0 damage except on weakspots
Monsters cannot be Stunned
20% more Monster Life
Monsters deal 15% increased Damage
Monsters have 20% increased attack speed
Monsters gain carapace armor everywhere except the weak spot
Increased Monster movespeed
Ground effects like burning or poison scattered through the map randomly, maybe even a lightning or “holy” effect which drains toughness or stamina in exchange for bonus damage
No healing stations, but medpacks heal the party to full OR players very slowly regen hp (1% every 4 or 5 sec)
Locked boxes with guaranteed supplies in a few places through the map, opening it summons large horde, patrol, double monster etc.
Storms of chaos or winds of magic randomly appear through the map, spinning like a small, harmless cyclone, then collapsing and applying a random buff or debuff to all units within
Okay time for my skeptic joke, I’d love to be able to at will throw mutations like these on to spice up the gameplay loop, but sadly it would probably result in too much information bloat and reading for our primal ogryn brains, and would probably create more crashes and bugs than are financially viable and logical 
just a joke, trust me bro xdd