Satan will show up at some point.
He's probably been laughing his ass off watching his brethren pick off the humans, but with Alex now set to fulfill his destiny, Ol' Scratch will step in to finish the job Gabriel started, with the Legions of Hell either joining Gabriel's forces, or forcing an Enemy Mine between them and the humans.- Alternately, he'll be the one to ally with the humans against Gabriel, like in The Prophecy.
- His existence is confirmed in the 1st season finale, when Michael mentions that "The Son of Morning" was born alone.
- he's referenced again in Season 3 episode 11 Bewilderment of The Heart and might make an appearance soon.
- Jossed with the cancellation of Dominion
The mysterious figure that healed Michael was Raphael
The Archangel Raphael is mostly associated with healing and is the patron saint of doctors. If there is anyone with the know-how to heal Michael’s wounds it would be Raphael.- According to a deleted scene, it was Noma who healed Michael.
Uriel is working for Lucifer
Related to the first guess above, this would explain why Uriel appears to be Playing Both Sides, and it would be an interesting way to bring in the devil, by having him start as manipulating everyone else.- she's working with Arika but for what end is unknown. She dies offscreen in the pilot of season 2.
The messages in Alex's tattoos aren't for him
They're actually messages he's supposed to be giving to others. "Beware of those closest to you" was actually meant to be a warning to Michael about Uriel and Becca. "She died for you" was confirmed to be for Senator Frost. Everyone is misunderstanding the purpose of the tattoos; they don't make Alex God's champion, but His messenger. God is in Hell
Because the angels would never look there. Uriel will return
In a show where angels body surf all the time, her season 1 actor not returning presents little obstacle to Uriel reappearing in another host. Bonus points if that host is either a) Arika herself or b) male. The Gunman (played by Hakeem Kae-Kazim) is Lucifer, who is also the protector of Mallory and the savior of William Whele
Michael's reaction to the dead Eightballs suggests that the Gunman has something to do with the fire at Mallory, and William's description of his rescue in the desert sounds similar to what he did when he saved Alex and Noma. Lucifer exists in this universe, and the rift between him and the other Archangels could mean that Michael is not aware of his abilities. The series wasn't actually canceled
God decided that that particular universe had gone too far Off the Rails.