Possibly not following rules: a literal 'dream game', i.e. lucid dreaming projected onto a computer somehow (for those people who never remember their dreams). this would be cool but probably isn't possible just yet.
More strict: A game that evolves over time as players do things. like Cleverbot.
Say the game has just started, and all you can do is move a small dot around a black void.
After a certain amount of time, the program will choose a random piece of code or ten and smash them together, thus adding a new element to the game; or it might remove one if there are too many that don't play well together. Added elements will probably depend on what the player does (e.g. death will lead to a different set of rules temporarily, 'heaven' possibly; surviving the dangerous situation that caused it might lead to a new game type or just continue onwards)
There could probably even be a random music/sound generator of some sort.
