For a long time, the Shazam! movie was going to depict a showdown between the eponymous World's Mightiest Mortal and Black Adam, one of the superhero's greatest adversaries. However, plans changed when it was announced that Dwayne "The Rock" Johnson, who has been attached to play Black Adam since September 2014, will instead first appear as the Kahndaqian anti-hero in his own movie first. There will come a day when Shazam and Black Adam will clash on the big screen, but for his first theatrical outing, Shazam will instead face Doctor Thaddeus Sivana. That said, the Shazam! trailer released this past weekend did tease Black Adam in the DC Extended Universe by alluding to the Council of Eternity, the group that Black Adam destroyed in the New 52 continuity.
In the middle of the Shazam trailer, Billy Batson successfully gets onto a subway car before the bullies chasing after him can board it, but just like in the comics, there's more to this subway car than meets the eye. In an instant, the other passengers disappear and Billy is transported to the Rock of Eternity, where the mysterious Wizard awaits to make the youth his new champion. This is how Shazam/Captain Marvel's origin story has always unfolded, but as you'll see below, the Wizard in Shazam! is surrounded by six empty chairs. If the comic book lore is being followed, then the other members of the Council of Eternity once sat in those chairs.
In the New 52 continuity, the Council of Eternity represented the seven mythologies of Earth, and it was these wizards' job to protect the world from evil and pass judgement on those they believed were guilty, such as the individuals who became The Phantom Stranger, The Question and Pandora. However, this alliance came tumbling down with the rise of Black Adam. In this version of the main DC universe, The Wizard had originally selected a Kahndaqi boy named Aman to be his champion, and Aman wanted to use his powers to help people and rid the slave masters who ruled Kahndaq of their evil souls. But Aman's uncle, Adam, was so consumed by anger and his need for vengeance that he stole Aman's power (it's strongly implied he killed his nephew) and became Black Adam so he could dish out his own brand of justice. At a later date, Black Adam killed most of the Council of Eternity, leaving the Wizard as the only survivor. Judging by those empty chairs, presumably a similar scenario happened in the DCEU.
Because Black Adam's backstory will no doubt be explored in his upcoming movie, it's unclear if the character will be referenced at all in Shazam! Still, the fact that director David F. Sandberg and the crew put those other chairs in the Rock of Eternity suggests that while there's no guarantee the Council of Eternity will be mentioned either, the organization did exist, and the Wizard is the only one left of the group. Although the DCEU seems to be steering away from "crossovers" following Justice League's underperformance in the theaters, Shazam and Black Adam are intertwined, so their movies are obviously going to be connected.
My guess is that the Council of Eternity will be mentioned in passing during Shazam!, while Black Adam, assuming we learn about the eponymous character's beginnings in Kahndaq thousands of years ago, will feature the full Council of Eternity and possibly show how all but the Wizard died. From there, ideally we'll see Shazam and Black Adam clash in Shazam! 2, but even beyond that, including the Council of Eternity can help broaden the magical side of the DCEU, which has already been touched on in Suicide Squad and will explored deeply in Justice League Dark.
Shazam! will work its magic in theaters on April 5, 2019, and although Black Adam doesn't have a release date set yet, one of the film's producer, Hiram Garcia, recently said that Adam Sztykiel's script is impressive. If you're interested in learning what else the DCEU has in development, look through our handy guide.