As every Supernatural fan (as in Castiel fan) I am devastated by 15x18. Cas went in one of the most meaningful ways we've seen on the show, but still, I'd prefer him not going at all.
I think there is still a tiniest sliver of hope. Misha Collins said Cas' ending was very "final". But is The Empty THAT final? Cas still exists in the Empty. We have seen characters coming out of the Empty. We know Empty has beef with Billie and God (probably); so...what if?
What if at the very end Empty lets him go for having Billie or for the Winchesters sorting the situation, of what if Jack does something (I know he's powerless now, but we don't know the circumstances, he might get his powers back)? We've seen it happen, we know the Empty can be "reasoned" with - Sam showed us that.
Of course; the prerequisite is the Winchesters surviving the end, otherwise it'd be pointless.
It's most likely not going to happen, this is Supernatural after all, and most of "clues" we have point to this being Cas' ending, but it's not completely crazy to hope that he might get out at the very end.
The producers and writers and cast all know how to "work" the fans, so maybe they just want us to truly believe that THIS is it, while it might not be...
Or maybe I just don't want Cas dead.
Also please note that we cannonically know Dean't can't have "happy ending" with Cas dead. It's been shown over and over than Dean is unable to get over Cas' dying, less alone if he considers it his fault - and he definitely will consider it as such.
So unless the writers plan to kill or otherwise get rid of Dean, they kinda have to bring Cas into the game again.
They're probably kill Dean though.