One of the biggest additions to the DCEU with the release of Zack Snyder's Justice League was the reveal that the character of General Swanwick, played by Harry Lennix, was actually the Martian Manhunter all along. While the Manhunter only had a brief cameo in the movie, fans responded positively to his appearance. When asked during Justice Con about appearing in future DCEU projects, Lennix suggested a detective movie centered around the Martian Manhunter.
"It would be great to see him as a detective. There are some iterations where he is a detective. Even if he is still Calvin Swanwick by day. To actually have him do some detective type work where he can use his special skills. We can see that diplomacy can be a kind of superpower. When you are a member of a cabinet where maybe world-ending fights in the sky might not be the most effective or practical way to deal with it. He could use his skills as a detective and his intelligence to avoid or avert some kind of catastrophe"
In the comics, the Martian Manhunter is an incredibly long-lived alien who has been stuck on Earth for a long time. Using his shape-shifting abilities to live as different people over the years, he makes use of his telepathy and other superpowers to solve crimes in secret, before making his public debut as a member of the Justice League.
As Harry Lennix points out, even though Swanwick is a part of the Government, his shape-shifting abilities could allow him to moonlight as a host of other characters as he uses his detective skills to avert a catastrophe without anyone in the general public ever coming to know of it, as has happened many times in the comics.
The one problem is that the J'onn J'onzz lacks any recognizable villains to feature in his solo movie. But Lennix has a solution for that as well. According to the actor, he would be less interested in taking on giant cosmic threats than the Joker, played by Jared Leto in the DCEU.
"I'd like to take on the Joker in some way, you know what I mean? I'd pick the Joker rather than those kind of other-worldly ones. The Joker is just a nutbag, who would be very interesting to take on."
It remains to be seen if the calls to #RestoretheSnyderVerse allow for a movie someday featuring the Martian Manhunter going up against the Clown Prince of Crime in a gritty detective thriller.
Zack Snyder's Justice League stars Ben Affleck as Batman, Gal Gadot as Wonder Woman, Henry Cavill as Superman, Amy Adams as Lois Lane, Jason Momoa as Aquaman, Ezra Miller as The Flash, Ray Fisher as Cyborg, Jeremy Irons as Alfred Pennyworth, Diane Lane as Martha Kent, Ray Porter as Darkseid, Ciarán Hinds as Steppenwolf, Jesse Eisenberg as Lex Luthor and J.K. Simmons as Commissioner Gordon. The film is currently available on HBO Max.