1. A throwing knife HUD tracker. Sometimes it's easy to lose knives once thrown. Also, allow players to pick up knives automatically, similar to health items.
2. A 50% camouflage or 'fade' effect for Elite Guards that ends after death.
3. A mutator that allows Undead (mummies) to explode into gibs like Flesh Zombies in the upcoming Survival Mode.
4. A jagged pixel or PS1-esque filter to complement the others.
5. A mutator that places exploding barrels, 'Nazi superweapon powerups' (increased damage, health, etc), or additional mounted machine guns/turrets for the player to use in levels.
6. A 'Fanatic' mutator that causes Soldiers to relentlessly pursue (never fall back) the player and a 'Loyalist' mutator that causes Soldiers to crouch/remain stationary (never move, but can still shoot) after spotting the player.
7. A 'Bleed Effect' realism mutator that causes the player and Soldiers to have a 15% chance of bleeding and spawning blood decals when shot. This can be stopped by using health items.
8. An 'Enemy Type' filter that determines the chances for specific enemy types to spawn in levels, such as more or less dogs, Elite Guards, etc.
9. An 'Ammo Realism' option that causes gun-wielding Soldiers to eventually run out of ammo. When this occurs, they either retreat or attack with a knife.
10. A 'No Hit Reaction Or Weapon Kickback' option, allowing the player to shoot or be wounded without jerking.
11. A 'Bullet Penetration' mutator, causing bullets to 'pass through' and damage enemies behind and in front of each other.
12. A mutator that allows the player to lean and shoot.

Actually I got an "Invisibility" thingy-do-thingy during a longer wave, like around wave 22 ish of the castle keep. I don't even remember seeing it or picking it up, it just suddenly said "invisibility" and though the soldiers still followed where I went, they didn't shoot or attack me. It was limited time, as it wore off... but I never even knew this was a thing..? New perk or? Either way, it saved my ass as there was 30+ soldiers and I ran out of ammo -- needed to run to the ammo spot to refill, and this made it possible.

Would be nice to have the option to zoom-in with mounted MGs, like your other weapons.
And the ability to dismount them (can be done with pressing `use` key when standing in front of it, from the side of the barrel)

Fixing the dogs` ability to open doors, maybe)

Fixing the dogs` ability to open doors, maybe)
Yah, because zee Nazi dogs are of sooperior breeds, and can open zee doors. *Adjusts monocle and brushes off iron cross.*
Ehehe. ;)

At least they can`t climb ladders. Though, some real dogs can climb when properly trained.