On further review, this idea doesn’t work very well as an MMO. The spirit of the game would be compromised by min-maxing competitively against other players and there are significant issues with what happens to player-owned assets when they log off. The game would be far more immersive in a single-player mode with an (perhaps mostly co-op) online multiplayer mode.
Here are some fairly broad design considerations I’ve derived from the concept:
No world map. If you chose an intelligent mutant, you can make a map if you can find the materials to do so—ink and paper may be quite difficult to find, but carving into tree-bark might work. The quality of the cartography depends on how perceptive and intelligent your mutant is.
Combat doesn’t have to be complicated. This game is about survival, not war. There needs to be enough of a combat engine to allow the player to defend himself and hunt for live game if need be. No vertical growth is necessary, though it may be nice to have. Most mutants won’t last very long. This game is more about experimentation than it is about building up an uber character.
A basic social model will be sufficient. The mutant needs to be able to express some simple sentiments. “Go here”, “kill this”, “I don’t have a weapon”, “I’m friendly”, “leave me alone or I’ll throttle you.” The chance of two mutants being capable of spoken communication and actually speaking the same language is infinitesimal, so we don’t have to worry about RPG dialog conventions.
Most gameplay is interaction with the terrain and environmental features—not necessarily NPCs; a dynamic and living world is the focus.
There will be story, but there won’t be exposition. The player, in search of food and water or maybe just to mess around, will stumble upon ruins of lost civilizations. Through ruin exploration, the player can implicitly learn about how the civilization was structured and what led to its downfall.
Interface will be a big challenge. If the interface is poor, all the interactive world systems in the world won’t save the game from floundering in uselessness and unplayability. This point is as important as most of the design points in the game, because without it the game cannot even limp towards success.