Top Ten Things Writers Need to Remember When Writing the Martian Manhunter
8. Give him back his secret identity.
Why? Well, I'm glad you asked, and if you didn't, I'll tell you anyway.
Giving J'onn J'onzz his secret identity of human John Jones accomplishes several things. First of all, it's the easiest way for him to observe humanity directly, and by assuming the guise of a human, it's the best way to show the differences between humans and a lone Martian. Whenever I hear J'onn waxing tragic about feeling alienated from humanity when he's in fact sitting far above Earth on the Watchtower, I feel the need to roll my eyes. How can you feel alienated from something if you never spend any time with it? It just rings hollow. To feel truly alone, you need to be around lots of people different than you.
Second, having J'onn assume a secret identity of a human opens the door to all sorts of juicy story conflict. Now, besides having to save the day, he's also got to keep a lid on his secret--that's more to juggle, and therefore, more interesting. The constant threat of having his identity exposed is a wellspring of conflict and keeps the reader in a constant state of suspense. There might be people threatening to give away that secret:
Detective Comics #243
Or J'onn might be tempted to give away that secret himself:
Detective Comics #246
It's a very classic conflict, and there's a reason it's stuck around for so long: from a storytelling standpoint, it just plain works, and it never gets old. That's why Clark Kent is still around. It also works wonders for effectively showing his alienation. It's so efficient that it only takes two panels for us to understand his dilemma. (You learn efficiency pretty quickly when you only have six pages to work with, I suppose.)
Third, having J'onn live amongst humans will deepen the character by allowing (or not allowing, depending on who it is) him to connect personally and professionally with humans. Letting him have some acquaintances and deal with the quirks of daily life on Earth will only help readers identify with him. Plus, it's another source of conflict: how close will he allow himself to get to his human friends? What will happen if he loses one? Also, seeing the people he cares about threatened by a villain is a classic weakness to be exploited, and J'onn J'onzz is the type of character who is defined by his weaknesses, not his strengths. Just don't have him do anything stupid and pointless while he's down on Earth, like, you know, live as a cat or something. (Wait, what? That actually happened?)
Finally, having J'onn live among humans will show how much he cares about us Earthlings. I mean, he's always going on about how he loves his adopted world, and he genuinely does. Well, let's see him put those words in action and get down in the thick of it, rather than stand off in the corner and watch everything from afar. Watching things from afar gets kinda dull after a while, both for you, and the reader.
In short, let's see John Jones come back. After all, in his early appearances, that's how he was billed: John Jones, Manhunter from Mars.