Jonn Jonez has potential.
He's a lonely lovecraftean horror whom has an overwhelming desire to do good.
That premise could drive stories for decades.
It even comes with a standard format that has been used from everything from the Fugitive, the Pretender to the Littlest Hobo. Heck the Zeta Project might give a base as well.
Here's a base structure.
1. Stranger comes into town tracking and pretends to be just a dude.
2. Stranger is Jonn tracking an unsolved case or mystery. Bonus points if it comes from his own past in some manner.
3. Jonn sets up things so that the punishment fits the crime, taking special care not to try too hard and to spend too much tiime on it, as any # of enemies are nipping at his heels.
4. Jonn berrates evil doer as his trap is sprung.
5. Jonn disappears into a crowd, as if he never existed.
Remember that stupid storyline where Superman went on a road trip.
with Jonn it would actually work.
and you can showcase parts of the dc universe. Themyscira, Atlantis, Gorilla City, even Apokalips would put a spin on the formula.
Now as for a rogue's gallery, it's pretty simple.
1. Cademus: They would be the main chaser as this "unseen monster" would set off Amanda Waller's button's hard. You could create a group of operatives (skilled and metas) who have done their homework and are prepped accordingly.
2. Alien infestations: Characters like the Reach, Intergang, and the Krolotean are one of the reasons he went under the radar. You do him like a boogymen to body snatchers. but shadow wars often cast a bright light.
3. Justice league: Something bad happened that was unforgivable to his former members causing him to go into exile. While the big guns are somewhat ignoring it (Batman is specifically obfuscating), some of the b and c members are particularly butt hurt allowing for hero vs hero battles.
A great foil that would also develop that character as well is Firestorm. He's powerful enough to force Jonn to pull out a ton of tricks and his powerset could kill Jonn. Plus the multiple minds acts as a partial defense against his telepathy.
4. Himself: One of the reasons Jonn Jonez works under the radar is if he ever cuts loose at full power, the madness takes over. If superman sees a world of cardboard, Jonn sees a world of action figures. He has to work to stay humble to avoid becoming one of the greatest threats to the planet.
It not just that. He's kinda callous and lonely. Pull a doctor who and give him a companion that acts as the pov character. They hang around for a story arc and then, he let's them go.
Sometimes he just disappears, leaving them to reflect and potentially grow in their own right
Sometimes he sets them up for a better life,
Sometimes they end tragically.
and in one case, he accidently makes a monster that hates him with everything he/she/it is.
5. At least one fire based character: Let's use that weakness against him.
6. Martian artifacts: One of the reasons he went into exile is a # of items during the 19th century are being unearthed. These items are tainted artifacts that usually result horror story ending for the wielder.
It's relatively easy to take one of these artifacts and build part of his rogues gallery.
7. Tharks: There is a community of Tharks that have been regenetically engineered to survive on our planet and are a tough bunch of goons. They have adapted ancient technology and have become planetary raiders.
Tar Tarkas did too good a job civilizing the Tharks, and they combined their brains and brawns to be a threat big enough to go toe to toe with some of the bigger threats.
and they remember the oppession of the green and then the white martians all too well.
They are a polar opposite of Jonn in that they are not subtle and play up their power for they are afraid to be powerless.
8. Lovecraftean horrors: I would add an element where part of the decline of the martian race was their dealing with stuff that is absolutely alien and incomprehsible. Perhaps the mi-go /martians fought a war thousands of years before both went into decline. Perhaps a shoggot is actually a corrupted martian.
9. White Martians: I would use these sparingly as they are more a capstone, much like Zod is to Superman. But the race that killed Mars continues to live like an infestation.
and give it a face. Even the Daleks have a few members that are distinctive as a face.
But I kinda like he idea of a little from column a and b for his powerset.
His telepathy, shapeshifting and density control is easy to use and mastered to act as a scapel, rather then a sledgehammer.
However, if he bulks up to superman level, he starts drawing on forces enscribed on his dna bringing the true horror from within and can potentially be a justice league buster in that form.
It's like summoning Cthulhu with at least a fighting chance due to the fact he is literally fighting himself from within.
That way it also explains why he's somewhat weaker as a hero and why he's usually meditative and soft spoken most of the time. He's worried about losing control.
Even gave it a title: Martian Tales - Stories to Shock.