A few weeks ago at Pro Tour Magic Origins, a new change to the mulligan rule was introduced. Dubbed the "Vancouver Mulligan" by many, the main change to the rule was the addition of "any player whose opening hand has fewer cards than his or her starting hand size may scry 1" to the existing mulligan operation.
Our goal with changing the rule was to reduce the number of games that didn't actually feel like games, and to give players more agency when faced with the difficult choice of starting a game down cards. Previously, when players had to take mulligans, too many of those games would be very lopsided, with one player barely participating before losing. We tested this new rule—and several other variations, as detailed by Sam Stoddard in this article—internally for months before deciding to move forward with it at the Pro Tour.
Now that we have observed how the mulligan rule was used at the Pro Tour and have received a lot of positive feedback regarding the change, we are making it an official addition to Magic's comprehensive rules as of Battle for Zendikar, meaning it goes into effect worldwide starting with the Battle for Zendikar Prereleases and will be implemented on Magic Online starting with their online Prerelease events.
In the meantime, you can watch the new mulligan rule used in the 2015 World Championship, which takes place at PAX Prime in Seattle, Washington, on August 27, 28, and 30. We are making a special exception for this event as an additional showcase for the rule.
The new mulligan rule in its entirety:
103.4. Each player draws a number of cards equal to his or her starting hand size, which is normally seven. (Some effects can modify a player's starting hand size.) A player who is dissatisfied with his or her initial hand may take a mulligan. First, the starting player declares whether or not he or she will take a mulligan. Then each other player in turn order does the same. Once each player has made a declaration, all players who decided to take mulligans do so at the same time. To take a mulligan, a player shuffles his or her hand back into his or her library, then draws a new hand of one fewer cards than he or she had before. If a player kept his or her hand of cards, those cards become the player's opening hand, and that player may not take any further mulligans. This process is then repeated until no player takes a mulligan. (Note that if a player's hand size reaches zero cards, that player must keep that hand.) Then, beginning with the starting player and proceeding in turn order, any player whose opening hand has fewer cards than his or her starting hand size may scry 1.