Justice League: 10 Ways Deathstroke Could Appear In The DCEU
7. He's Part Of Suicide Squad Mark 2
In Suicide Squad's mid-credits scene, we saw Bruce Wayne and Amanda Waller meeting for a nice steak and exchange of metahuman files. The main drive of the scene was rather pointless - Bruce already has Lex's QuickTime videos on the future Justice League, so why did he need to go to ARGUS for the same info? - but in its last moments it did reveal two key pieces of information; Waller knows who Batman is and Bruce Wayne doesn't approve of Task Force X.
This is a conflict that isn't going to lie and will inevitably come up at some point in future movies, possibly in The Batman but more likely in Suicide Squad 2 (which - fingers crossed - takes a tighter, Assault On Arkham approach to the story), although it could be set up across the other Bat-centric films. And that's where Deathstroke comes in.
Slade has been a member of the team in the comics, and if he's a free man is the perfect person to be keeping tabs on Batman's own team for Waller. It may weaken him as a villain - Suicide Squad suffered a lot from forcibly saying basic anti-heroes were bad guys - but works as an introduction and is the type of cross-movie storytelling the DCEU has been leaning towards.