Fire rate multiplier and bullet damage multipliers. Maybe you'd have to get rid of Double Tap or at least get 1.0 over 2.0. Though increased power scaling might mean you could make the zombies tougher or throw different opponents in.
Maybe an ultra rare wonder weapon boon. Maybe some of the dead ops arcade drops, it'd be cool to get a pet chicken shooting with you. After all I feel like dead ops has a lot of roguelike elements to it already,

I got to like round 30 and went through about just as many boons so if I said anything that already exists my bad. Overall I thought the map was great and I think if this idea was on a map that wasn't a straight away it'd be crazy good. Having an easter egg with a boss fight would be awesome, though I know that's gotta be difficult to make. If you do make a full map with this concept, just having some panzers or any other special zombies would spice it up as well.

Ammo Regen/Bottomless Pouch
-Self explanatory, gun refills ammo slowly, if you get more than one it increases the speed of ammo regen each time

Death Sprint/Cow Catcher/Pain Train
-When you start sprinting into zombies, it will kill them, active for a few seconds, time increases with each duplicate boon

Area of Effect bullets/BIG BULLETS
-If a bullet hits a zombie, the damage is spread evenly among nearby zombies, alternatively you could have the first zombie take full damage and all nearby zombies take a percentage of that damage, each duplicate boon increasing that percentage

Shell
-Have the effect of a shield on your back

Gentle Jeff
-A permanent Turned zombie that is VERY slow, getting dupes adds cosmetics to Jeff (Could be random things like those Arrow-Through-The-Head headbands, or a top hat or googly eyes, you could have a LOT on Jeff)

Increased Chance for Powerup Drops
- Self explanatory, duplicate boons increase the chances

Whoops!
-Zombies have a chance to drop QEDs with all their possible associated effects, duplicates have no effect (QEDs are VERY risky nades)

Very Long Knife
-Infinite melee range (If the crosshair is on the zombie, the melee can hit it), duplicates have no effect

Bionic Upgrades
-Gain the ability to use the Exo jump.