As you would if you were him, Zack Snyder posted a photo of himself on the Justice League set and wearing a Batman gauntlet to Twitter, causing nerds everywhere to scream, "WHAT'S ON HIS TABLET?" The answer is Deathstroke. Deathstroke is on his tablet. Here's the tweet:
— Zack Snyder (@ZackSnyder) September 29, 2016
And here's a blown up and reoriented look at what's on his tablet. You know, like sane people make and pin to walls with highlighted documents and string connecting them:
And it looks like Slade Wilson -- with eyepatch, so no helmet -- talking to someone. Feel free to speculate away.