Reason 1: A Villain Centric Film
Audiences have craved for a movie focused on a comic book villain rather than just your average superhero. Now granted we will be getting a movie of this sort about a year from now with David Ayer's Suicide Squad, and there have been rumors that Slade Wilson could make an appearance.
But even if DC's deadliest mercenary does make an appearance, it's doubtful that he would get a large amount of focus or development. And this leads into the next reason.
Reason 2: Great Story Potential
Deathstroke is such a diverse character and there is more than enough to go off of in a feature film. He's not just a highly skilled BadA! assassin who can use up to 90% of his brain capacity and has an intellect on par with Batman. He's also a father. One of the things I love about the current Deathstroke comic series is the way it handles the relationship with his children. He wants to reconnect and make amends with them but they just push him away. At the beginning of Vol. 2, his daughter Rose is so unwilling to that when he gives her a note he asks her to give to her brother Jericho if she sees him, she burns it. Including that development in a film could give Slade's character some depth. You would sympathize for him and understand why he does what he does. Everything he does, he does for his family. Add to that the story of how he received his enhanced senses and strength plus tons of awesome action full of all manor of swords and firearms and you have one epic movie. Of course there's much more to that but it would take too long to come up with the full story.
Reason 3: Better Establishment For Future Appearances
If Deathstroke received his own standalone movie, the development we discussed in the last point would mean whenever we see him again in another film prior, we already know who he is and we don't have to explain his whole backstory and we can get on with whatever is going on in the film he's appearing in, whether it be in a Batman or Green Arrow film or any other DC property.
Reason 4: More Attention For An Underrated Character
While Deathstroke is a favorite among hardcore DC fans, he's most likely lesser known to general moviegoers who are more familiar with (groans in irritation) Deadpool. Deathstroke is far more deserving of a movie which is part of the reason I'm writing this post. Giving him his own movie would introduce audiences to a character just as BadA! but much more dignified and interesting than Deadpool.
So now lets talk casting. Now there have been some ridiculous rumors that Scott Eastwood is playing Slade Wilson in Suicide Squad. That obviously can't be true do to him being far too young. Granted at the beginning of the new comic series, Slade is mysteriously de-aged but it's doubtful the writers would use that plot point this early on. My top picks would be Sean Penn or Viggo Mortenson.