Credits scene

This would be very anti-climactic. But still, it would be better than some of the alternatives on the list.

Thus, while Suicide Squad was the first DCEU flick to have a credits scene, who is to say that the following movies won't continue the trend? After all, the meeting between Waller and Wayne can be tied to almost every movie planned for the DCEU. Having Deathstroke in a credits-scene would hint at even bigger plans for the character than we would have believed (think about Thanos' first credits scene).

Still, to show the actual scene so early on would ruin it for the fans. But I guess it is better than revealing one of the main antagonists of the entire movie.

Who knows? Maybe Deathstroke will see an opportunity rising after Steppenwolf's failed invasion. And maybe he will be shown infiltrating the Justice League base. Why? It will be revealed in subsequent movies, of course.