An agent of Steppenwolf

As the Justice League teaser has revealed, and as Wonder Woman herself said in Batman v Superman: Dawn of Justice, Darkseid has tried to conquer Earth in the past. And that is why I believe that, since ancient times, the ruler of Apokolips has infiltrated all sorts of agents on our planet.

Deathstroke could be one of them. And his task would be to locate the Mother Boxes. In this way, Steppenwolf would only have to come and collect them. It's a pretty thin scenario and it would also reduce Deathstroke to just a simple secondary (if not tertiary) character.

But I guess this is still better than other alternatives. In this way, he'd still have an active role in the plot of the movie. And also, a connection with Cyborg would be established.

After all, the Terminator is a mercenary who can be hired by the highest bidder. And since Darkseid rules over an entire planet, who can be more powerful and wealthier than he is?