Survivability vs Synergy
Given the idea that less survivability = more desire for cooperation, give each perk their own Survivability vs Synergy perk option that is automatically chosen based on whether or not the player is playing solo or with teammates. This way, synergy is increased with less detriment to solo play, and devs have more opportunities to create or fine tune perk balance.
As examples:
Zerk's Survival Option (solo play only)
This perk option functions the same as the KF2 Skirmisher option.
Zerk's Synergy Option (team play only)
Take Only What You Need. Sell the Rest. Similar to picking up another player's weapon, this perk allows the player to pick up ammo and armor but then "sell" the unused portions for teammates. Teammates interact with the Zerk like they would for free ammo/grenades from other perks, turning the Zerk into a golden retriever of pickups. Zerks get bonus dosh when another player "buys" the unused portion.
For example, a Zerk has 80% armor and picks up a nearby armor drop, bringing the Zerk's armor up to 100% and carrying an additional 80%. He then brings it over to his teammate, who has no armor, so that his teammate can approach, interact, and take the 80% remaining armor. It costs the teammate nothing but the Zerk gets a dosh bonus.
Medic's Survival Option (solo play only)
Carry Only What You Need. There's no point in having healing dart attachments to your gun if you have nobody to shoot them at, so leave those behind. Every perk weapon now weighs 1-2 pounds less and medic weapons alt-fire is disabled.
and/or
Experimental Ingredients. The more damage you do to zeds between syringe uses = your syringe gives you an increasingly potent damage buff on the next use.
and/or
Poison Darts. Those darts that would normally heal teammates now cause poison damage to zeds. If zeds are killed while the poison is in effect, they explode with a healing gas, similar to a weak Healing Grenade.
Medic's Synergy Option (team play only)
Great Minds Think Alike. All Syringe and Dart effects get a boost the more players are carrying Medic perk weapons.
and/or
Pharmacist. Teammates' syringes get temporary stat boosts on their next use every time the teammate interacts with the medic during combat. The medic gets extra dosh for "selling" these drugs.