See I understand that Martian Manhunter is a fan favorite character and Cyborg is still viewed as a side kick type of guy but there are some elements which makes Martian Manhunter somewhat unfit to be a member of Justice league in the "movie format" so here me out.
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Let's talk about the character POV, which is the heart and soul of any character through which film makers define and differentiate those characters, so basically who is Martian Manhunter the simple answer is,
He's an alien from another planet who also happens to be the lone survivor of his race and eventually he adopts Earth as his home and humans as his family.
Sounds awful lot like another character right, a guy who dresses in Blue and Red.
Thus the basic underlying element of both Superman and Martian Manhunter are similar and writers had a hard time figuring out on how to craft a story with two characters with similar elements, which eventually lead to taking Martian Manhunter out of Justice league in New 52 series of DC comics, which was a boon in disguise for the character, because in the Justice League Animated series, Martian Manhunter's role was toned down to a great extent and he was made the keeper of Watch Tower who rarely goes to field missions and was often portrayed little weak, all of this was done to differentiate him from Superman but on the other hand keeping him away from the membership of Justice league worked out wonders for the character especially in the New 52 series, because now Superman doesn't steal away the moments and he's on his own which really made this guy one of the most creepiest and powerful character in New 52 DC universe.
It's like writers really made a more realistic version of himself, it's basically like he lives in secret and only the government and Justice League knows about him and only if Martian Manhunter feels like if it's okay for them to know about him otherwise he just wipes away the memory of him from their minds altogether ! And he really plays a lot from behind the scenes pulling all the strings in a really creepy way with his powers of Telepathy and shape shifting. As a matter of fact he influenced a lot of people in the government to draft and execute new laws/plans like the creation of Justice League of America, for example and no one knows about that !
Long story short, Martian Manhunter has a different approach to do things, more like a lone wolf than a team player/everybody's guy but sometimes it may mean controlling and reading the minds of 100 people at once and making them do what he wants ! Yes the Martian Manhunter of New 52 is that paranoid. And I believe it's because of the reason that he's a telepath and shape shifter, I mean being a telepath and shape shifter, he goes through the collective evil/negative intentions of the entire population on Earth and that would drive any guy crazy but I believe it would make Martian Manhunter a more darker character who would view mankind from a really different perspective, like this guy.
And these type of guys, even though they come under the category of "good guys" and doesn't do any villain or criminal kinda stuff they really aren't team players and they prefer to work alone and do things on their own and manipulate the events/people to achieve the desired goals (which often benefit the safety and security of Mankind)
So I believe this is how Zack Snyder and WB would be treating the Martian Manhunter in DC Cinematic Universe, yes he will appear in DCCU no doubts about it but he won't be a Justice League member but that doesn't mean he won't be helping them down the line !
Now to the next part of this article, where I tackle the situation of why Cyborg is an excellent addition to the roster of Justice League !
Again let's about the character to understand him better, all that Victor Stone (Cyborg) ever wanted to be, is to become a great Football/Rugby star and have a normal life with a girl and boom one accident changes him into walking dead (no pun intended) I mean he's half man and half machine who is robbed off of his humanity and his dreams and that's a terrible thing to happen, especially when you are damn close to realizing your dreams yet he somewhat tries to forget it all and move on with his new life, and now that's a trait or an experience that's totally relatable and will connect well with moviegoers.
And secondly he's the only 24X7 super hero in the entire comic book world, I mean almost every comic book character has a secret identity or normal life of their own where they do other things and blend in with regular human population but this guy has somewhat of a miserable life, because even if he wanted to, he cannot blend in and move along with regular people yet he's constantly being updated about every single email, tweet, bank transaction, phone call, SMS and every other form of electronic communication medium from every corner of the planet 24x7 no matter how strongly encrypted they are!
so it's like you are running away from your ex but you are getting phone calls, tweets, messages from him/her all the time and you cannot do anything about it, pretty much relatable right ! And because of all these experiences [Cyborg](movie:1043082) became very hardened and mature who is quite calculative of everything he does even when he's one of their youngest member.
So from a character perspective, Victor Stone/Cyborg brings a new layer to the team dynamics of Justice league which Martian Manhunter "may not" bring, which in turn makes the whole Justice league even more awesome.