I've always wondered how on earth Deathstroke fit as the villain of Affleck's Arkham set psychological thriller? Some kind of Knightfall scenario, maybe? Hope one day that screenplay leaks or at the very least we get a fuller picture since all we really know is Slade, an Arkham setting and apparently Tim Drake (Johns says Dick Grayson's actor from Titans got the role off an audition for Drake in Affleck's Batman.).
To answer the threads question, for a hypothetical new Slade Keanu Reeves has my vote. That one eyed child grooming weirdo belongs as the big bad of a Titans film though, not Batman.