Again, I think it will be some kind of side-quest.
Bringing Joker back wouldn't be worth the surprise element. Even if people didn't expect it I doubt anyone's reaction would be "Oh, what a surprise".
But, as with the Johnny Charisma: Live advertisements, I expect that Batman's paranoia about Joker still being alive will be addressed as part of a side-quest or even in the main story. Maybe fear toxin will play a part in it. But bringing him back wouldn't have the impact to make it worth it ("Oh, another Joker story, yay!").
He's gone, and I only see one way of Rocksteady topping that surprise (that would be killing Batman, probably at some point before the end of the game).