Random idea #1
An animatic about Crowley's life story.
He was originally the archangel Raphael, loved by all the heavens. Gabriel feared that he would soon take his place, and sought to destroy him.
Raphael had no idea, of course, and was bonding with a few angels who were meant to fall, according to the ineffable plan. Gabe took this as a chance that he was dishonoring the heavens by being too close with the soon-to-fall angels (they took care that Raphael doesn't find out about angels falling). And so, Raphael was sentenced to fall.
And we have Crowley.
(Guys please don't tell Aziraphale it'll break his immortal heart into smithereens)