Patty Jenkins' Wonder Woman is a beautifully self-contained story in its own right, but it's obviously leading to something much bigger. In fact, the home release of the film has added an epilogue that helps tie the events of Wonder Woman in with Justice League later this year. In the movie's new scene, Etta Candy sits down with Sameer, Charlie, and Chief to tell them about their next mission to recover an ancient artifact -- and as the scene concludes, we see that the "goal" of the mission is to retrieve a Mother Box. The clip ends with the crew looking over papers, and we see this sketch:
Etta teases the presence of the Mother Box when she tells Steve Trevor's old crew that they are going after something old and powerful. Then, the clip confirms the existence of the Mother Box, which we know will be crucial to Justice League. As a standard fixture in the cosmic side of the DC Universe, Mother Boxes are a primary trope in stories featuring Darkseid and Apokolips, and we already know that three of them will be the MacGuffins of Justice League. If three of them are on Earth when Steppenwolf finally arrives, then having Wonder Woman's old comrades recover one of them in the early 20th century is a nice way to tie things together and set up the next story.
It also seems to make sense because it potentially explains the scene from the initial Justice League trailer in which we see a group of knights (presumably in ancient times) burying a Mother Box in a European forest. The bombs from World War I helped bring it to the surface.
If that particular Mother Box falls into the hands of the American military, then it stands to reason that it could eventually end up in the hands of Silas Stone at S.T.A.R. Labs for research. From there, the scientist could use it on his gravely injured son -- which eventually results in the creation of Cyborg. Between that Mother Box, as well as the rumored presence of two more in Atlantis and Themyscira, we now potentially have every Justice League Mother Box accounted for ahead of the movie's release.