I was just playing DK and was playing some of the bonus/easter egg levels We should definitely have some of these So lets come up with strange rules for easter eggs.
For example one level in DK is only accessible when it is full moon
Another posibility is to make the game much harder between 03:00 to 06:00, this was for example the case in some of the Mortal Kombat games (I am not sure of the specific time intervals, but it was sometimes very hard at night, and the computer made fatalities almost every time )