Magicball: the simplest version would probably be as follows: "During each player's turn, that player can change up to one rule of the game at instant speed, except for rules regarding win conditions or rules already changed in this game by other players." The last clause is necessary to keep people from playing tug of war over one card or board state. So, for example, if I cast a Baneslayer Angel, on my opponent's turn he can change the rule about flying so it now works like phasing and my angel won't be around next turn. On my turn I can change first strike to work like double strike so when I finally do get my angel it hits even harder (or change the phasing rules to make it less bad for me, or just change flying to shadow or horsemanship, but let's keep this example simple), but I can't just change flying back to flying. The game is likely to end when players realize that "up to one rule of the game" includes this rule, and they start allowing multiple rules changes per turn or something. But then, it always was clear that it takes a certain mentality to play Calvinball and enjoy it.