Season six, but from Castiel’s POV.
We see the war in heaven, and the strain of what’s actually happening. We see the angels mourning each other and the order. We see the chaos of Michael getting lost in the Cage.
We see Cas ( or maybe some other angel) attempt to summon Michael from the Cage to help put back the order, but only end up with half of Sam instead.
We see angelic true forms. The depth of their power.
We see Cas learn, slowly, why he prefers humans to angels. We see his siblings turn on him and attempt to kill him over and over. We see power struggles, and anger and the pain. We see Sam and Dean become a refuge, not a problem. We see Sam and Dean attempting to help, and failing.
We see angels threatening Cas’s only safe place and safe people (Sam, Dean and Bobby) We see events in history that Cas took place in.
We see people’s personal heavens desecrated throughout the war.