That's the one thing I'm still afraid of, and I can't decide if it's actually likely to happen, or if it's just what would happen if Sera was still in control.

You know what would be really great, though? If they started the storyline like this, you know, Dean being back, no Cas, Dean refuses to talk about what happened, Cas only in flashbacks, etc. And then after a few episodes there is the big reveal - but hey, we all 'know' already that Cas sacrificed himself for Dean in Purgatory, right? - and it turns out that it's something else entirely, something actually unexpected.
If someone could pull this off it's Jeremy Carver, the man who gave us a hilarious Trickster episode, just to reveal at the end that the Trickster is really the archangel Gabriel.