Deadshot, Deathstroke, Amanda Waller and Rick Flagg are musts for me. When I think of the Suicide Squad those are the characters I think of first.
I'd prefer to see Harley Quinn's transformation first before she ends up in the Suicide Squad though I understand wanting her in a movie like this because she would make it more interesting. I just don't like the direction some of this DC Universe seems to be going in with regards to Batman, they seem to be skipping alot of interesting and important history if some of these rumors are true about Batman being in his mid 40's, Carrie Kelly being Robin, Harley being separated from Joker already, skipping Dick Grayson transformation into Nightwing, Jason Todd already dead, Jim Gordon likely being retired, heavy Dark Knight Returns influence etc. etc. etc.
I want to see all that on screen, some of those stories I've been waiting to see forever in live action form.
And I don't care what anyone says, you can never over-use The Joker, he could be in every single Batman film as long as he's written well and played by an actor that can pull it off.