If I could design an MMO, completely from the ground up and I had full control, what would I do? Easy. Zombies.
First off, there would be only one world, and the world, in effect, would be as large as the real world. It would not be a zoneless game, but for most people it would feel like it. When you log in for the first time, you create a character, no classes, just a person (using the obligatory super cool character creator with eleventy billion combinations of body parts and textures and clothes), and you choose where you are going to be from (a nice world map with glowy red dots for player populations, so you could decide to be where there are lots of people, or where there are no people, or somewhere in between). You would also choose a “player type”. This “type” would decide if you are issued a house or if you are given a list of dwellings of appropriate survivor capacities that are currently open to new members (if you pick a type for a size of group to which none is available, a new one will be created for you and new players will begin joining you shortly). In game a player can always choose to change their “type” by simply leaving their current group and joining a group of another type (or if you have decided to abandon people and go it solo, you have to clean out a house and bar the doors for safety).
Welcome to a world populated by endless zombies. You and your group (or just you) need to implement and maintain defences as well as gather survival supplies, food, fresh water, broken pipe replacements, clothes for the winter, a way to stay cool in the summer, weapons, etc etc. And of course the zombies need to be killed, because zombies tend to bunch, and if you don’t keep an eye out you’ll be surrounded and starve to death before you know it. Zombies tend to walk, but some will run when they see food. Rarely do they think, but once in a while they’ll be accidentally crafty. Log in, survive, log out… but don’t forget to make sure you’ve got food before you leave, you might starve while you are gone (working in groups helps, if your mall has 24 residents you can afford to not log in for a month as long as the rest of your group keeps you fed, but be careful, if you don’t pull your weight, they might feed you to the ghouls).
When you are logged out, your character is an NPC in defence mode. Unless other members of your group are online to manage the perimeter, its assumed you will warn people trying to break in and kill them before they succeed.
If a place gets too overpopulated by players, not only will you run out of zombies, but you’ll run out of food, unless you start farming, but farming (unlike your home) isn’t safe and you might get robbed. Get enough people working together and you might rebuild a city, at least until people stop logging in… dead players becomes zombies, and your city might be destroyed from within.
And what about player death? Well, a player can only have one character at a time. If they find out friends or coworkers play, they can always travel to them… if they can survive the trip that is. Death is death, and you have to roll up a new character, but without levels in the game, the only thing you’ll really have lost is your home and your possessions (given to your groupmates if you had any, left for scavengers if you didn’t).
What can you do besides fight zombies and survive? Well, anything you want to really, as long as you are also fighting zombies and surviving. Rebuild a PC and get a working satellite dish and some power and you can hook up to the remains of the internet and communicate with others, even play games… or play games with your group mates or other neighbors when you have them over for tea (or they get cut off while scavenging and needed a place to hide until daybreak). Depending on bandwidth and licensing issues, run your own TV station or radio station… publish a newspaper. Rebuild a car and go for a drive (be sure to lock the doors).
Essentially, Second Life… but with zombies!