I don't know if its just me seeing this, but they do have a fenced off area next to The Joker. I'm not sure if it's a big enough area, but that would be a perfect spot for a Zamperla Giant Discovery.
That really would be a perfect fit, but there's so many places it could go. Such as, Old Country, Rolling Thunder's former spot (Entrance in Golden Kingdom), or on the Lakefront.