Major League Baseball and the Major League Baseball Players Association are kicking ideas back-and-forth at each other right now for the 2019 season. Ken Rosenthal of The Athletic notes more than a few of the proposed ideas between the two parties. One of the ones of note is that the players are pushing for a universal designated hitter beginning in the 2019 season.
Since the 1973 season the designated hitter has existed in the American League. The National League has never used it other than in the World Series road games, and in interleague road games during the regular season.