Zombie Apocalypse MMO:
5 classes: Bad ass, Mechanic, Medic, Psychopath, Worker
Vehicles: Mechanic can make vehicles that help carry around party members I.E. bus and motorcycle.
Weapons: Workers make different weapons for Bad Asses and Psychopaths to use. They can also help Medics heal party members.
Healing: Medics can heal party members using bandages and vaccinations. If a player gets heavily damaged, then medics can get workers to help do surgery.
Plan of attack: to complete most objectives, you need to make a plan of attack. This is the work of Psychopaths. Psychopaths can also use stealth to distract and attack zombies and other apocalypse gang-parties.
thats my rough breakdown of what i have thought of so far.
EDIT:
Bad ass: damage/tank
Worker: damage/healing
Mechanic: damage
Psychopath: damage
Medic: healing