Some fun feature and mutator suggestions.

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.