Update: The London Mulligan will become the official mulligan rule with the release of Core Set 2020 in July, 2019.
On Weekly MTG, Wizards R&D member Ian Duke announced a new mulligan procedure that would be used only at Mythic Championship London in April.
At Mythic Championship London, players will draw seven cards every time they mulligan. When a player is satisfied and decides to keep their hand, they will put one card from their hand back on the bottom of their library for each time they chose to mulligan. Players will no longer scry after mulliganing.
This is a dramatic departure from the current mulligan rule, first used a Pro Tour Vancouver in August 2015 and thus called the Vancouver Mulligan. Currently, when a player mulligans they draw one less card every time they choose to mulligan. After a player chooses to keep their hand, they are allowed to scry one card to the top or bottom of their library.
Duke said that this was intended an experiment for improving the mulligan process in order to reduce the number of non-games due to excessive mulligans. This comes on the heels of Luis Scott-Vargas mulliganing to four in Game 5 of the finals of Pro Tour Guilds of Ravnica in November, essentially giving him no opportunity to win the game. If the test goes well, Wizards will look to implement this in all formats around the summer release of M20. If the test goes poorly, they will go back to the drawing board and keep the current mulligan rule.
Duke also said that if the new mulligan rule sticks, Wizards will take a look at the Banned & Restricted list and reevaluate cards that might be a problem or take too much advantage with the new mulligan rule, such as Vintage Dredge.