When plugged into a computer and launched, the game uses the computer's MAC address to procedurally generate an environment unique to that particular computer. It then connects to a central server to download changes to that environment made by other users who have played the game on that computer, as well as uploading any changes you make so that future visitors would be able to find them - kinda like PMOG/The Nethernet but across physical systems rather than websites.
Internet cafes and public computers would become collaborative world-building projects, all without ever having to read or store files on computers you do not own/have write-access to. Players could go on quests across multiple computers in multiple locations to collect items or complete objectives. Finding items or creations in hard-to-reach worlds would become more of an achievement.
This has been a rambling game idea, brought to you by Gold Prognosticus.
never know tho