The shift is a modern day baseball "Moneyball" sort of concept. Essentially, a manager will move a fielder over to where the batter is more likely to hit it. There are 4 main types of shifts, each of which I have detailed below.
Normal Defense Shift
This is the basic form of defense, where a team will put every fielder in the "normal" position. There isn't much variety here, and this "shift" takes up about 90% of all at bats (rough estimate).
Hulking Lefty Shift
In baseball, there are some players who pull the ball so much, that teams will put 3 infielders on one side of the field, and only 1 on the other side. In this case, 3 players will go in between first and second base. The outfield also shifts towards the direction of where the infield is. This shift is used for hulking lefties like Joey Gallo, Daniel Vogelbach, and Nomar Mazara.
This is the same shift as the hulking lefty shift, except that 3 players will move to the gap in between second and third base.
Mike Trout Shift
This shift is where the outfielders go back as far as they can to try to stop Mike Trout. Spoiler: Nothing can stop Mike Trout.
So what's next in defense? I believe that it will be a spin-off of a shift. I think that what's next in defense will be someone covering the middle, sort of like an outfielder playing very, very shallow. This would prevent players such as Joey Gallo going right back up the middle.