what is season 8 about?

Nobody knows what season 8 is about. It hasn't even been a full Monat since they stopped filming season seven and, since they're getting a "new" head writer, I seriously doubt they have very much written for it yet.

As of right now, we know
-Sam and Dean are separated (Dean in Purgatory with Cas)
-Crowley has Meg and the prophet kid
-Jensen is set to direct the first episode
-Jeremy Carver is going to be executive producer
-The Zeigen was moved to Wednesdays
-The first episode is tentatively set to air sometime in October.

And that is literally all we know right now.