An introduction

One of the best scenes from Batman v Superman: Dawn of Justice is when the Dark Knight is finally introduced. To make that happen, Zack Snyder cleverly decided for the Batman to go against a literal nobody "“ we didn't know who the hero was and the director revealed him, without wasting any important villains.

So Justice League could go on the same path and show us how the Batman keeps on fighting crime in Gotham. Now we already know what he can do, so having him go against some nameless goons isn't good enough. And here enters Deathstroke, as a brutal mercenary. Such a scene won't just showcase Batman's skills, but it will also show that Gotham is still a city you'd better think twice about moving into.

Who knows? Maybe this scene will also introduce the Flying Fox. And who says that Deathstroke cannot appear afterwards in other DCEU movies? Even if he ends up locked up after encountering the Batman, there is always the possibility of escape.