Very random idea but ... what if every round once when you have 2 or more perks the game takes 2 of them (at random) and combines them into an additional perk ability.

I'll explain ...

Juggernog + Quick Revive = Faster recovery from red screen
PhD Flopper + Electric Cherry = Dolphin dive/sliding explosions also electrocute zombies
PhD Flopper + Deadshot Daiquiri = All damage done by dolphin dive/sliding explosions is done to zombies heads
Stamin Up + Quick Revive = Can choose to self revive yourself at any point (solo)
Mule Kick + Double Tap = 3rd weapon slot fires damage as if weapon is pack a punched.
Double Tap + Electric Cherry = 2nd bullet (fires 2 bullets as normal) is electric charged.
Speed Cola + Stamin Up = Unlimited sprint and almost no reload time?
Widow's Wine + Deadshot Daiquiri - Widow's wine grenades wrap around zombies head (insta kill)

It might make players very OP on a few rounds but maybe this could be done?