Devil Hulk is suppressing the other Hulks
I believe that in a way the Devil/Immortal Hulk isnt just a father figure to Bruce or the Savage Hulk, but to all of them. In other words, what he basically did to the professor, the green scar, and the other missing Hulks was akin to putting them in a corner or grounding them, but still having access by watching the adventures of Bruce and the Devil Hulk, the only exceptions being possibly Hydra hulk(because he is dead) and the guilt hulk.
- Further theory if this is true: the reason why Devil Hulk has "put in the corner" Green Scar, it was because of the latter's failings as a father. Considering that Devil Hulk sees himself as a father figure Brian Banner never was and how Green Scar's fight with Skaar ended the former realizing he is acting no better than Brian (not to even mention the fate of Hiro-Kala or how Gren Scar essentially dumped Lyra on Jennifer's lap), it feels like something Devil would take an issue with.
The One Below All is consuming Hulk from the inside
This theory was already mentioned as an alternative explanations where did Professor and Green Scar go in other parts of this article. Adding it to the list for the sake of consistency.
Amadeus Cho is correct why Professor and Green Scar have gone silent.
Amadeus' theory Banner's two most heroic Hulks have gone silent as a protest against Bruce and Devil Hulk's action is another possible reason for their disappearance.
The Green Scar has been consumed and/or replaced by The One Below All.
The Green Scar is shown blatantly lying to and manipulating the Savage Hulk, beats a chained-up Devil Hulk for no given reason, and brushes off the idea of freeing Joe. Additionally, he doesn't actually do anything to assist the Savage Hulk, instead sitting back and prodding him.
Probably the most concerning pieces of evidence that something is very wrong are that when the Green Scar briefly takes over to kill Xemnu, not only is Savage Hulk left with no memory of what happened but Hulk's veins bulge grotesquely, spikes erupt from his arms and back, and he refers to Earth as "his planet."
All of this is very much out of character for him, given that he's actually one of the more heroic and reasonable Hulks. The fact that the Breaker of Worlds looks like the Green Scar and shares both his title and motto, how the Leader tells Xemnu to "look deeper" after he boasts about having control of Savage Hulk's mind, and how he refers to this Hulk as "coming up to meet you" further support this possibility.
The One Below All is Satan.
It was revealed in a Fear Itself tiein that the Marvel Universe's true Satan is immeasurably more powerful than any previouslyseen demon, has been absent from Hell for a very long time, and no one, not Dormammu, Marduk Kurios, Mephisto, anyone, has any idea where he is or whether he is even (still) real. Seems like a prime candidate for "is the antithesis of the One Above All (basically God) and was trapped below the multiverse and only able to interact with any particular universe by latching onto souls to use as avatars, plus wants to destroy everything".
- Power-wise, he seems more closer to Angra Mainyu, however he already exists as a weaker character so everyone just assumes The One Below All is Satan if they think he exists.
Cosmic Rays are the One Above Alls equivalent to Gamma.
The series has had an emphasis on duality since day one, but one thing has been missing is a balance to The One Below All and its emanations. The series has been dropping hints about what that balancer is: Cosmic Rays/the Power Cosmic. The first hint came in issue 24 when it shows the Fantastic Fours spaceship ride occurring at the same time as the Gamma Bomb, hitting Banner with both energies. When the flashback to the creation of Galactus is shown, the same tac sound is heard, before the sentience of the Cosmos gives the become a new form of life speech. At the flash-forward of the end of the universe, the Sentience gives the same speech despite no there being no radiation in sight. This seems to indicate that the two are the same.
The connection to the One Above All is proven by Fantastic Four 511. When Ben nearly dies and is on life support, Reed uses a device to follow his consciousness. They find Ben in his human form (like how gamma mutates who die and go to the Below-Place are untransformed) stuck outside of a door. After his dead brother explains whats going on and Reed and Ben bond, the door opens with a blinding light similar to the light that comes from the House of Ideas.
Upcoming solicitations show that Ben and the U-Foes, a group of Cosmic Ray-powered villains will appear. After the events of issue 39, the Hulk might be turning to the other side of the coin.
- This point is further supported by the fact that in their first appearance, the U-Foes die of Power Incontinence. Yet, when they next appear no explanation is given as to how they survived things like blowing up or being shot into the vacuum of space. If the Green Door has a counterpart that is opened by Cosmic Rays, it would be the perfect explanation for how they came back.
The One Above All will confront The One Below All at the end of the series.
By the time of the Ninth Cosmos, the Hulk possessed by TOBA has succeeded at emptying creation, and it looks like every other cosmic entity is dead. Given the seriousness of this, TOAA will finally get involved. As His only weapon is love, He will defeat TOBA by restoring Banner/the Hulks' personalities and allowing him to be reborn, banishing The Anti-God out of him and allowed Him to restart things as the Tenth Cosmos. Assuming, of course, the events of the book don't Ret-Gone this Bad Future away.