I would want to play some kind of post-apocalyptic MMO, something that was inspired by and drew elements from Mad Max, Akira, and Fist of the North Star. Survival would be the big theme in the game. You'd fight bandits/gangs, mutants, etc. Some classes I'd like to see would like an engineer/gunslinger class that could customize their weapons to alter their abilities/skills. A mystic/martial artist class that would rely on martial techniques/"chi" (like Fist of the North Star). A half-mutant/psychic class that had telekinetic/mental powers (like Akira). A vagabond/samurai class that would kind of be like Michonne from the "Walking Dead".
Some areas would be like vast deserts with sporadic encampments with roaming nomads, cities on the water in areas have been flooded by polar ice caps melting, underground cities, ruins of abandoned cities.
A large part of the story would be trying to find out what the hell happened to the world.
I guess the closest thing would be a really good Borderlands MMO.
