Ben Affleck shared the following video on Twitter, showing what appears to be test footage of the DC comics assassin Deathstroke. There is no indication what film this is from, but we can assume it’s from the currently filming Justice League, though it could very well be from the upcoming Batman solo film.
Deathstroke played a major role in the first couple of seasons of CW’s Arrow (played by Manu Bennett), but he has really only been mentioned since then. If the character is in fact going to join the DC Cinematic Universe, his absence from the show would make sense. The Suicide Squad were also retired from Arrow for the same reason.
https://www.youtube.com/watch?v=TLZB69FUM2c