Mostly, he's talking about the "insane" developments coming between now and the end of the series, which DC has promised will be shocking and status quo-changing.
"I can tell you there are definitely some insane moments in the conclusion and some crazy things happen so, I've always been braced for it as something that isn't designed to just shock for the sake of shocking, but is the culmination of the story in a way that feels organic and surprising,” Snyder explained to The Huffington Post. “I can't remember who it was, but I had a teacher that gave me a writing quote when I was a student that said 'Every ending should be both inevitable and totally surprising,' and I completely believe that. Hopefully this is an ending that feels organic to the story we've been telling, and when you get there you'll say 'I can't believe this just happened,' and at the same time, you'll feel that it's a cumulative, horrible thing out of everything that's been going on."
He also examined a bit of the psychology of the story, which re-casts and redefines The Joker for the first time since The Dark Knight:
“Joker is coming at Batman from a place of very personal terror for me. And a place of personal guilt, and also a place of great personal anxiety,” Snyder told Popmatters. “So I think he is coming after Bruce in a way that is getting at a very, very personal story for Bruce. Because as much as Bruce loves the Family, you could make the accusation that in some ways he wishes he could stop worrying about them. Does that me he wishes they didn’t exist? Of course not. Does that mean he doesn’t love them? No, of course not. But that’s the way the Joker interprets it. The Joker says, ‘You’ve become slow and weak, and you’ve become nothing of the Batman I used to know. So deep down I know you’re unhappy. And I’ve heard you think this almost (not in a supernatural way, but just in a kind of figurative way),’ So he says, ‘Let me grant you your wish, because I’m here to serve you. And we’ll have so much fun afterwards.’”