Scarecrow, either locked up in Arkham, Batman and Gordon pass his cell while they are walking to meet with someone in Arkham..something like that.
or have him still on the loose, trying to use his fear toxin on Joker.
Fox, he has to be in it, he is running Wayne Enterprise, he has to give bruce new toys.
Zsas, maybe if Joker has thugs Zsas could be one of them, or he could just be in prison.
Ra's, back from the dead, ready to destroy Gotham and it's protector, this time he brings his daughter. and perhaps brings along Poison Ivy, having the same kind of role that Scarecrow had, she wants to destroy Gotham because it's a concrete jungle....