My opinion is no, he shouldn't be. But maybe...just a little.

We know Joker will be in the film. And by be in the film I hope it is a very small part. Just one scene. I'm thinking, as many do, Harley leads them into a break in of Arkham because they need some info for their op only Mr. J can provide.

After cleverly maneuvering all the guards and snares the squad approaches the Joker's maximum security cell. We hear. We hear the Joker's voice denying something, we hear a slam, a crack, we here a deep baritone demanding information...

The squad turns the corner, looks into the Joker's cell and find...Nothing. Just Joker slumped down against a wall with a broken bloody nose. No one else in the room. They ask "what the hell?" Joker laughs, as only Joker can in these moments and replies "Oh that was just Batsy." Then the squad looks a little awed and perplexed and the movie carries on.