The Insurgent Faction side in Co-Op has their Suicide Bombers which made playing Co-Op WAY more interesting.
For playing against the Security Faction, there's no special enemy to worry about when playing as the Insurgent faction. Sure the Security faction has their Gunship and Minigun Helicopter Support, but with the ground soldiers, they are all normal foot grunts.
I have some ideas for special class enemies for the Security Faction in Co-Op (mostly based off of PAYDAY2):
RIOT CONTROL (Riot Shield wielder with a Pistol as their only weapon. Can charge and knock down allies with two hits)
TASER SPECIALIST (Has a taser to shock allies stopping them in their tracks. Enemies can take advantage of tased allies by shooting at them. Being tased will be in a short instance)
MEDIC (Can revive a few downed enemies)
JUGGERNAUT (Full Body Armor using a Machinegun)
SPEC OPS (Based off of INS's VIP/Black Security where they use silenced highly-modified weapons and flank more)
SCOUT (Only uses pistols, but moves and shoots faster)
I don't really have that many original ideas for special enemy types for the Security faction, but these ideas would be a start