Just heard Manfred on ESPN discussing umpiring issues. He said that they are testing two systems in the minors, one that provides the ball/strike calls to the umpires in their ear (the so-called robo umps), another that allows teams to challenge umpire calls. If they challenge, the umpire receives the correct call via an ear piece.
He said the later is attractive because it keeps the human element in the game and that the challenges are resolved in seconds. There would likely be a limit on the number of challenges.
It sounds like the challenge system might be here before long.