Although little is known about what role the Martian Manhunter will play in the upcoming HBO Max mini-series, Zack Snyder's Justice League (March 2021), we have now received confirmation from both the director, Zack Snyder, and the character's actor, Harry Lennix, that the alien from Mars will indeed appear on screen in his full, green-skinned glory.
In 2019, the first of what is expected to be a Shazam-lead trilogy starring Zachary Levi was released, however outside of a brief (headless) Superman cameo, we haven't seen him interact with the greater DCEU yet. And while Ryan Reynold's Green Lantern days seem far-behind him, many fans hope to still see this character explored within the DCEU movieverse created by Zack Snyder. While this has been hinted at in Justice League behind-the-scene photos and castings, albeit, with a different actor and unrelated backstory, the Green Lanterns have already been introduced in the theatrical release's opening sequence.
Despite all that and until we see these characters "unite" officially, we do get the chance to envision, make-believe and pretend. Using official promotional material from the Green Lantern movie (2011), Justice League movie (2017), Shazam movie (2019), as well as fan-art combined with official character shots of the Martian Manhunter from the CW television series Supergirl (2015-current), we are able to visualize what an expanded Justice League team could look like.