*Taken from CoD*
You get a group of people (from a solo group to a group of four and lets just say we're in a group of four at the moment)
Your group leader goes to a NPC (Named whatever) and the NPC will take you to the custom zone / Zombie map. (Probably add more maps as time goes on)
You are teleported to the central room of the map. You start at level 1. Each wave you go up one level. Each kill you get and how much you heal the doors (talked about in the next paragraph), you get points. You can use these points to talk to doors. These doors are different. These are objects you talk to in order to pay points and gain access to new rooms. Also, you can use the points to buy guns off the wall, guns from the mystery box, the upgrades/buffs, and the pack-a-punch upgrade for your weapon.
Zombies start coming from outside the building (Lets say the map is Kino der Toten, the theater) to hit on the doors. The doors will be attack able by them only (Like wintergrasp destructible walls) but players can heal them. You get your points, open up doors, get weapons, go through the place looking for the object to "turn on the power" (probably set you in a new phase once you turn it on) and now you can use the teleporter to get to the pack-a-punch. You get ~30 seconds? to do whatever in there and then it takes you back. The teleporter has a 10 min cool down.
The mysterybox is an object that randomly spawns in different rooms. There can only be one at any given time. It will have 5-7 charges. After it reaches 0 charges it will disappear and re-appear in a new room. When you talk to it, it will give you a random weapon for whatever amount of points (a set amount).
You can go and talk to objects, Soda Machines(COD thing, need ne obj ofc), and specific ones give you specific buffs. Such as: % Increased health, % increased dmg, % increased armor, % increased ranged attacked speed, % ranged arp, and probably some others.
Now, the weapons. Will be guns only, with 2 or 3 melee. There will be different categories. Pistols, Assault Rifles, Shotguns, Snipers, RPGs, and melee. With each category you get different spells. Also, there is no auto attack with these weapons. (Side note, you can use these spells while moving, but LOS LOS LOS) DMG varies from weapon to weapon. You can also only have two weapons, not including the melee.
~Pistols: You can dual wield pistols. If you do, you get "Dual Shot" where you shoot with both weapons. You also get "Single shot" where you just shoot with one weapon. You also have double the ammo when you have two equipped. Depending on the weapon depends how fast you can use this and the range of the weapon. (The cooldown is faster or slower) Maybe as the starting weapon have a special type of pistol that has never ending ammo.
~Assault Rifles: You just get "shoot." If your gun is semi-auto, it shoots one bullet at a time. If it's a burt it shoots three at a time. If you have a fully automatic it will bring up a cast time bar (kind of like you're using arcane barrage) and it will last until your ammo in your "clip" is all gone. Depending on the weapon depends how fast you can use this and the range of the weapon. (The cooldown is faster or slower)
~Shotguns: While using a shotgun, you use a "Shoot" spell. It's like the mage spell "Dragon's Breath." It does damage in a cone in front of you. Depending on the weapon depends how fast you can use this and the range of the weapon. (The cooldown is faster or slower)
~Snipers: All single shot. "Shoot" spell. Depending on the weapon depends how fast you can use this and the range of the weapon. (The cooldown is faster or slower)
~RPGs: You click your spell and choose were to place it. Like "Heroic Leap" that warriors have or "Death and Decay" Death Knights have. It does damage in the area that you chose. The size of the area, how fast you use it, and how far away of an area you can select depends on the weapon.
~Melee: For example knife. You start with the knife. You slash with at the target enemy. You have to be in melee range. You can get Galvanized Knuckles. You punch with them. It hits with physical damage and shock damage. Probably some other melee weapons.
Any who, depending on the wave you get to, you get a rewards. That's up to whomever decides that. I was thinking every three to five waves the reward gets bigger.
Not much else I can think of at the moment. It's all a little clunky, I know, but I believe it gets the point across.