It seems that Supernatural season 9 is the season of parallels. Garth revealed that Dean doesn't care where love comes from, that love is love. The chastity group at the church revealed that Dean wants to settle down in a romantic relationship. The Ghost Facers paralleled Dean and Sam's sibling relationship. The same occurred with Cas and Gabe's conversation in Meta Fiction. There was an episode in season 5 where Sam and Dean were said to be descended from Cain and Abel, which I think hints Dean might kill Sam in the finale using the First Blade. The following scene is my idea of the last scene of the finale, since you can tell it's a doozy from what Jared, Misha, and Jensen said in an interview:
"'Something happens in the finale that is a first we'll see,'" Jared told JJ on the carpet. 'I'd say don't miss the last 30 seconds of the finale.'
Misha told us, 'I can say that the fans are going to freak out. I think it's very chilling and disturbing. When I read the script for the finale, I was definitely shocked. I thought, 'Can they do this?' The last five seconds!'
'I'm going to one up [Jared and Misha] and say it's the last second,' Jensen said. 'The moment the screen goes to black. The last second. There are multiple characters involved. There's a predicament now.'"
Castiel looked at Sam's body that was crumpled on the floor of the Men of Letters bunker. Dean stood over the younger Winchester with the First Blade, covered in the blood of Gadreel, Metatron, and now Sam.
"Dean..." The former angel whispered. He leaned against one of the tables, his grace slowly burning away. The older Winchester walked slowly toward him. "Dean, I don't want to hurt you." The green eyed man struck Cas on his temple. His legs crumpled, and he kneeled before the Winchester.
Dean raised the First Blade up in his right hand. Cas grabbed his left sleeve, only to have his right arm broken by the furious man. He screamed. Dean still hold onto Cas's right arm, and repeatedly punched him in the face.
"Please, Dean. Don't do this," Cas said between punches. Dean froze, arm up in the air, still holding the First Blade. "We've been in a similar situation before... The First Blade is controlling you, just like Naomi controlled me. You saved me then, and I need to save you. But... this isn't you, Dean. I need you. I... I love you."
Dean dropped the first blade. Castiel blinked slowly. He could feel his grace about to give out. "Yeah... I love you, Dean..." Cas collapsed, and Dean sat down next to him, cradling his lifeless body.
"Cas... Cas..." Dean turned around to shout to Sam, but saw that his brother was lying in a pool of his own blood. He burst into tears. "Don't do this to me, Cas. I love you, too... Wake up..."
And then in season 10, Dean would sell his soul (yet again, sigh...) to bring Sam and Cas back to life.
3BFFs does not own Supernatural. Interview is from Just Jared. If you go to the Supernatural Wiki, the link is on the home page with other links to spoilers for the finale. Links don't work for Fan Fiction, sorry...