Director Zack Snyder takes to the Vero app to reveal something many fans have thought all along, which is that Harry Lennix actually was going to be the Martian Manhunter.
Snyder released what appears to be a Justice League storyboard involving a scene between Lois and Martha, where Martha is really the Martian Manhunter and then becomes General Swanwick.
"This is something that you probably didn't know," Snyder posted.
Martian Manhunter storyboard:
Harry Lennix as Martian Manhunter fan theory proven true
Following Man of Steel, many fans thought Harry Lennix was playing Martian Manhunter due to two scenes.
One scene involved Superman getting interrogated where he breaks the handcuffs, and interestingly enough, Swanwick is the only person not to get scared and take a step back. Swanwick also remarks about Superman having revealed his true identity to Lois Lane, and asks, "Why won't you do the same with us?" Superman then replies back as he breaks the cuffs, "Let's put our cards on the table here, General." It's possible Superman was implying that he knew Swanwick's true identity as well.
The second scene is at the end of Man of Steel when Superman confronts Swanwick about being followed by drones. Superman tells Swanwick he has to do things his own way, and then Swanwick says, "Even if I were willing to try, what makes you think they'd listen?" Superman follows up with a seemingly cryptic response of, "I don't know, General. Guess I'll have to trust you." As I noted years ago, I personally thought that exchange between the two may have meant Superman knew Swanwick was a fellow alien - Martian Manhunter - and that's why he trusts him.
Back in 2016 actually saw Harry Lennix deny that he was Manhunter when asked, as he tweeted, "Not the Manhunter but someone else thought I should be. It would be cool."
So it appears as if either Lennix was playing coy back then, didn't know that he would become the Martian Manhunter, or Zack Snyder eventually agreed with the fan theory and was going to incorporate it in Justice League and the DCEU.
We can also assume that Martian Manhunter would have joined the Justice League.