Heaven's throne is empty.
Nobody in heaven above or hell below has seen or heard from god in centuries.
Satan is doing double duty in heaven, earth, and hell and learning it's not all it's cracked up to be, having lost passion for torturing and testing humanity long since*. Yet, he can't bring himself to truly take the throne, fearing that it may be a trap, that God's apparent absence has been one long gambit, and the moment he perches himself on the throne something very bad will happen.
The denizens of hell have not been informed, they still think heaven is more than the glorified gated-community retirement home it has become, since satan is unable to make a perfect society of spiritually renewed people, he plays it safe and accepts only those unlikely to ever cause trouble. The current state of things is unsustainable;
The armies of heaven and hell have been decimated from war and without god to create new spirits the population is dwindling, while hell reaches an unmanageable capacity of the damned. Thus, a plan has been set in place to end humanity entirely via apocalypse, and then the spiritual realm in its entirety will go quietly with it.
The only hope for humanity and the universe is if it can prove they can change, and improve themselves, and therefore up to the task of managing the affairs of life and death themselves. Charlie's idea for the hotel, which she's been obsessed with since birth, was planted in her mind by Michael to initiate this test of humanity's worthiness.
In this universe, the angels and demons are still sexymen but less extravagantly dressed, and more varied in body type, and their bodies aren't real anyways they're actually just the form they take to interact with human souls. in reality, they're depicted more similar to the angels from "It's a Wonderful Life", you get the sense that none of them take things super seriously but are still unfailingly dutiful as is in an angels nature, though without clear direction, they struggle to find duty to which they can attach themselves.
the twist is that god isn't dead, but has been in hiding, and satan was right, it was part of a gambit, but this is never spelled out, much less what the gambit even is. Only Michael may or may not be receiving orders from him, it's up in the air. God's intentions are unclear and he has no on-screen presence.
*Satan is going the Megamind arc of his initial reason for becoming evil being out of spite for god using him as a pawn, making him the scapegoat in his plan to grant humanity free will in the garden of eden (which he could not admit to himself that he wanted to do), but now that god is gone, and he has "won", he doesn't see the point anymore. In general the tone of the series is nihilistic in a vein similar to hitchhikers guide to the galaxy, where everyone, not just humanity, deals with the same petty inconveniences, frustrations and disinvested existential angst no matter how alien to the human condition they supposedly are.
