rule variations

One idea I'm toying with is the possibility of changing certain rules of the game, while the game is being played. The idea is that as soon as a player reaches a certain condition, he/she may change or add or subtract a rule (whatever condition they've met may determine what rule they can change). Rule changes, if good, may be added to a directory of possible rule changes. Good rule changes should significantly alter the course of the game (pattern). These rules may become the basis of a sort of grammar used for the symbolic language; they may switch the “topic” of a game, or somehow alter the meaning of the same topic.