First of all, this is not just my idea. A server was started (Necropolis or something like that) with this concept but never made it far, though I think it was a great idea and might go on to make a server like it. I've taken the theme but incorporated my own ideas and thoughts, so tell me what you think.
The idea was a server theme of the zombie apocalypse. The tutorial would start you off choosing a faction. There would be about 4 factions, each featuring its own community, unique weapons, and a base that holds off the zombies outside. There would be several NPC's giving missions to players, including sending them out to get ammo, food, supplies or even to protect the base for a certain amount of time while protecting the base from an incoming hoard.
After picking the faction, the player picks a class. Examples of classes would be Scavenger (finds more loot, and better loot, but with light armor), the Guardian (has a lot of health and defense, good at pushing back groups of zombies, uses shields and explosives), Fighter (mainly uses melee weapons to deal damage to incoming zombies, uses medium armor), and the Sharpshooter (who uses guns to kill zombies from a distance, but use light armor).
The community would be very close, with players needing each other to survive certain missions. Every so often, there would be a random zombie attack, where a hoard would invade the base's safe zone. Factions would be able to call for help from the other factions in which they would both benefit in either gralats or loot or something of that kind. Factions can also attack one another, in the case that they are enemies. The faction that wins the fight would be able to gain loot and supplies from the dead graalians.
Players could also go out on their own or with a gang (smaller group similar to guilds on classic) too go and loot gralats from the ground, trash cans, and other places. You would also find a lot of loot from dead zombies. While out there, when you shoot a zombie, the noise will attract more zombies to your location. This will make it harder to survive the longer you are out there.
There will also be different levels of zombies. The most common will be a regular zombie graalian, which drops common loot and supplies. There will be many moderately hard zombies, with different abilities such as shooting a gun or having a sword. They will drop guns very rarely (not super rare guns) and a moderate amount of ammo and supplies Then there will be boss zombies, which will be found in deserted skyscrapers and other such places. They will drop the rare weapons and large amounts of ammunition and supplies. It will take a large team to fight them, and each person would get a random drop from the boss zombie.
There are still many more ideas I have, but I'll leave you with the main concept.