Justice League: 10 Ways Deathstroke Could Appear In The DCEU
8. A Post-Credits Scene
After Man Of Steel and Batman V Superman both shirked the trend of post-credits scenes teasing the future of the franchise, Suicide Squad saw DC finally jump on the bandwagon with a mid-credits sting that tied into pretty much every upcoming movie (and why shouldn't they - it's not like the trope was created by Marvel). With that in mind, it'd be a surprise if Wonder Woman and Justice League didn't continue the trick, and Deathstroke is a pretty strong candidate to feature in the latter's; rather than playing any part in the plot, even in a minor capacity, he's instead just there to set-up a later appearance.
Revealing the content of the post-credits scene so far in advance would be even more outrageous than showing the movie's ending, although if Deathstroke is confirmed as The Batman's primary villain between now and the release then it still has the potential to be (somewhat) unexpected.
Crucially, it also explains why this is coming later on in Justice League's production and immediately ensures that the fan-favourite character's appearance won't distract from the more cosmic focus of the main film.