They're actually considering recasting Greenlee, since they're thinking of having her connected to the Satin Slayer in some form.
Josh Duhamel (Leo) is doing well on Las Vegas
, so I see very little chance of him returning.
As for the characters I'd like to see back, David and Brooke need to be back first. The show may have made the most idiotic decision in its entire history when they fired those two.