Shazam focuses on Billy Batson, who can become the hero known as Captain Marvel (not the Marvel incarnation set to played by Brie Larson) by saying the word, 'Shazam!' which grants him the powers of six mythological gods.
Johnson's character has long been the nemesis for the hero, but in recent comics, he has been on a path to change his nature and make amends for his past. And that would seem to be the angle here, with Johnson looking to find the fun in the DC universe.
Had a very cool and strategic meeting with the heads of DC about their entire universe. As a hard core DC fan, to get a real sense of the tonal shifts and developments coming in these future movies has me fired up. Something we, as DC fans have all been waiting for for a very long time. Hope, optimism & FUN. Even when talking about the the most ruthless villain/anti-hero of all time finally coming to life. Prepare yourselves DC Universe. #KneelAtHisFeet #OrGetCrushedByHisBoot #BlackAdam⚡️
So DC Entertainment's team, including boss Geoff Johns apparently decided that rather than consigning Black Adam to mere villain character status, they would let him breathe in a film of his own. Which is arguably understandable when you consider Johnson has been attached to play the role since 2008 and was officially confirmed back in 2014.
When either of the movies might hit is currently anyone's guess, especially given Johnson's busy schedule. But it certainly sounds like he is looking to power another potential franchise...