With DC releasing Suicide Squad and Marvel releasing Deadpool in theaters next year, one could assume and crave the release a Deathstroke movie to connect and compete (respectively). Here is how it should go.
The movie should start with his origin. He should be well into his military career, have his crush on Adeline, ally himself with Wintergreen, and get his serum.
The main goal should be similar to that of World Tour. In a fight to save Adeline from the League of Assassins, Deathstroke loses his eye and the criminals escape with her. The primary portion should be his search for her, meeting Vigilante in the process, and facing off against Batman briefly.
After Slade saves Adeline, Amanda Waller approaches him to recruit him for Suicide Squad.
Deathstroke- Ron Perlman
Perlman should reprise his role for this live-action portrayal.
Adeline Kane Wilson- Haley Atwell
I know she'd be exactly the same as Peggy Carter, but she did it very well.
Batman- Ben Affleck
Amanda Waller- Viola Davis
Billy Wintergreen- Christopher Eccleston
From his part in Doctor Who, Eccleston is perfect for an ally met in the military.
Vigilante- Rachel Nichols
She would handle this role very well.
Ra's Al Ghul- Ben Kingsley
Ra's Al Ghul is the DC equivalent of the Mandarine, but here, he should do it right.