My group almost always plays in a Grand Melee format, and to mix things up during the month of April we created the Wacky Roolz format. The players would agree to a new global rule before anyone drew any cards. These rules could be as simple as "Automatic Howling Mine" or "every card has cycling" or "any card can be sacrificed for two colorless to draw a card." This added some great spice to our game sessions and really caused us to bend our noodles coming up with rules. Our stat sheet even listed all the roolz used for the entire month. Try it with your group.