whether to go ahead and allow the divine power source, assuming that the old gods of Earth mythology returned with the return of magic, or to keep them away as with Dark Sun proper
I'd do one of two things.
The first is to just keep it out. The gods are long dead. There is no one out there to save you. The most powerful things are evil Sorcerer-Kings. This keeps the hopelessness and struggle of Dark Sun pretty intact.
The second is to use an apocalyptic pantheon, like the Norse, or the Mayan, Aztec, or Incan. Ragnarok and the bak'tun cycles are ripe for use on a dying world, and I think if you go with the Meso/South American stuff, it will jibe with Dark Sun weirdness very well. Blood sacrifices and hummingbird war-gods and feathered dragons are very evocative (though, honestly, I'm a bit partial to that whole mythscape

). I'd even be a little tempted to use the Book of Revelations from the Bible or the Qur'an's apocalyptic entires, but I'm a dork about that stuff.
what the hell the campaign itself will actually entail/be about.
Well, since you're already playing around with the idea of a "return of the gods," I might make it about that. Sacrifice, worship, and potential salvation at the End of Days.
Voiceover said:
The Left-Handed Hummingbird drinks our blood to fight the darkness. We know that some day, he will fail.
Our city was carved from the dry sands and steel bones of our giant ancestors who lived in the sky. It was beset upon by wilderness, and sorcerers, and death. It is still being carved, still being flayed from the flesh of the armies of our enemies, their blood staining the sands as dark red as the sun. Ours does too. The Left-Handed Hummingbird is our immortal king, and without our blood, he will die.
We give him our blood because he returns to us an unseen power. This is not the dark magic of sorcery. This is not the mental tricks of oracles and enchanters. This is not the brutality of the hermits of the wilderness, or the simple actions of soldiers. We call it the True Power, and we call it Sacred. In our Lord's name, we heal our soldiers {Clerics}. With our Lord's image, we protect and destroy {Runepriest}. Our Lord has assassins {Avengers} and soldiers {Paladins} who make war in his name. Through our Lord's words, we shape the world {Invoker}.
We give him blood, and he gives us unheard of power. Without the blood, he will die. When he does, we will eat his flesh, and become Lords ourselves.
So, through
sacrifice of some sort, the divine power source is being unlocked, creating gods out of lords out of mortals.
And this is where the PC's come in.
They may be characters in service to one of these "lords." Or they may be from a traditional city-state, perhaps even agents of a jealous Sorcerer-King. Or they might even be looking to become "lords" themselves, puzzling out the mystery of where this power is coming from, and how to give their own followers this.
The secret that they should discover, by the end of the campaign, is that this relates to the End Times that are near. Those whose souls are dedicated to a Lord might find life after the End of the World, in an Afterlife, something no one has ever heard of before. Perhaps in the sand-filled, steel-barred ruins of the Ancients, they find a Codex relating to the End of Days (perhaps they stumble on a ruined museum, or even a destroyed chapel with a Bible inside).
Their enemies include sorcerer-kings trying to quash this new source of power, and rival lords, and, of course, the horrible creatures of the wastes themselves, where the clues to this ancient secret lie buried.
This means that their final adventures will be a central question: with this knowledge, do they try to save the people of the world? And are they willing to be the cause of sacrifice, if it means giving people an Afterlife? Which ones get Saved? And how?
They may have the capacity to become gods themselves (or to advance the cause of their own god), but this should be a bit of a moral dilemma for them, a question that is reflected in the world. Everything is going to die some day soon. Do you let nature take its course, or do you let people harm themselves through fasting, through human sacrifice, through various abstinences, in order to have a shot at an Afterlife that even you (even as a god) are not entirely sure of the nature of?
You can bring hope to a hopeless world. You can give people a reason to live. You can save souls. You can become a god yourself.