Here's a little something I've wanted since Der Riese.
Think about it. You buy a perk, you complete a specific challenge for that perk, once it's completed, you see a small star above the perk symbol. You then go back to the perk machine, buy the perk again, and then that small star above the perk moves down and changes the symbol to the pro perk symbol, the star shaped one like in multiplayer.
The challenges could be:
Juggernog - Kill 250 zombies
Speed Cola - Reload 125 times
Quick Revive - (Multiplayer) Revive 10 Team-mates (Solo) Survive 10 rounds without going down (Can only be achieved after round 6)
Double Tap - Fire 2 Magazines into enemies without missing (Can only be achieved with automatic/3 round burst, bullet firing weapons.)
Stamin-up - Sprint 2 Miles Total
Deadshot Daiquiri - Get 100 headshots
Mule Kick - get 75 kills with each weapon
PhD Flopper - Get 50 kills using the perk
Then of course, all pro perks have a new effect. For example:
Juggernog - Survive 1 extra hit
Speed Cola - Switch weapons faster
Quick Revive - (Multiplayer) The player you revive keeps all pro perks for one down. (Solo) You keep all pro perks one down, but you lose quick revive completely.
Double Tap - You don't need to prime Grenades or special grenades, but if you ant to cook a grenade, you can still hold down the button. Eg, you instantly throw the monkey bomb, and you don't pull the pin on the grenade, but you can still cook it.
Stamin-Up - Unlimited sprint
Deadshot Daiquiri - Less Recoil on all weapons
Mule Kick - Hold one extra weapon, 2 extra grenades and one extra special grenade. And, if you lose the perk, you lose the gun with the lowest damage.
PhD Flopper - Larger blast radius and more damage.
I still haven't decided wether or not the perk should cost more, but I think that if you go down with the pro perk, you lose the pro effect, but not the perk. Pro perks would make the monkeys that steal perks less annoying as well, as they could only take the pro perk.
Hope you like my thoughts! Be sure to reply with any changes you think should be made!