Regarding Deathstroke... I don't think he's in this but I do hope we'll see him in the future. That's one character I would LOVE to see, but I'm not sure which way they will go with him. He could be older (as he's typically portrayed in the comics). I could see them going younger though, if they do his full origin. After seeing Fury, I really, REALLY would love to see Brad Pitt as Slade if they go the older route. He could be amazing. Plus, it seems like the kind of role where they could persue a big name.
If they go younger, well... Lately I've been championing Zach McGowan (Black Sails) for a number of comic book characters. I think he could make a perfect Wolverine or Lobo, or maybe even Batman, so Slade might be a good fit for him too. Come to think of it, Toby Stephens (also from Black Sails) might make a good older Slade.