From Ask Ausiello:

Question: Can you tell me anything about Supernatural‘s new season? —Bryan

Ausiello: I can’t be certain, but I think I just uncovered a significant clue about the overriding plot of season 6: Sources confirm to me exclusively that X-Files vet Mitch Pileggi is in talks to reprise his role as Sam and Dean’s presumed-dead grandpa on a potentially recurring basis.

From SFX Magazine issue #198: "There's also been talk in the writers' room about how to bring The Trickster (Archangel Gabriel) back."

EEEEE!!!!

Well, given his track record for faking his own death, it wouldn't be THAT much of a stretch.

Yeah, I could easily see "Oh, Lucifer killed the pet parakeet of one of those hotel guests shifted into my shape. I stowed away in my image on Sam's laptop and laid low until all the fireworks were over."

If Castiel can be brought back for choosing the right path, why not Gabriel, I say.

Here, here!

Or is it, hear, hear!

In any case - bring on Gabriel!!

Hmmm. Gabriel's powerful enough to travel through time, so theoretically he could show up again and be his pre-"Hammer of the Gods" self skipping forward a year or more.

Hell, why not? Shouldn't be that difficult to write Gabriel back in given all the plot devices they have available.

According to Fred Lehne, he will be returning as Azazel in Season 6. OMG!

Also looks like Corin Nemec will be in a recurring role as Sam and Dean's cousin.

Awesome, but, how?