Scenario #1: After fighting an epic battle, the brothers are left standing next to the Impala, bruised and bloodied, but alive and together. With everything that is said and done over the course of the seasons, they've come full-circle with their relationship as brothers again, just grown up and matured and progressed since all they've gone through. With after making some kind of typical Winchester crack, further indicating how all-right they, they get into the Impala and drive off into the sunset. Perhaps with the classic line of, "we've got work to do." Suggesting that while the show might be over, the Winchesters still ride on, fighting the good fight, doing what they do best.
Scenario #2: The brothers are fighting an epic battle, realizing that there are only two options, either repeat the cycle of sacrificing and bargaining or going out in a blaze of glory, saving everyone in the process. They choose the blaze because that's how the Winchesters roll, yo. This would obviously be an interesting scenario considering that they've been to Heaven before, and who knows? Perhaps they find themselves in Heaven after sacrificing themselves, and they see Mary and John there. Not memories, but their real souls in Heaven. Winchester family reunion after all this time, after all the turmoil they've been put through, they are finally together again. And not only that, but Dean and Sam going out together as brothers would also be symbolic to everything that's happened.
Never mind the fact that this fits perfectly for the show's unofficial theme song, "Carry On My Wayward Son" -- if anything, the series finale should fit for what the lyrics are suggesting, that their entire journey was worth everything and that they are finally at peace.
As for the fate of Castiel, I see it happening in one of two ways. After the War in Heaven is eradicated, Castiel either goes back to Heaven but still watches over the Winchesters (in either Scenario #1 or Scenario #2) because he is still their friend after all, or Castiel decides to stay on Earth and carry out particular missions and retelling the story of the Winchester legacy. Or something to that effect. It's still being thought out on that one, but I definitely believe that Castiel will continue to be in contact with Dean and Sam no matter what happens.
Highly doubtful any of this will happen, since the show and I aren't communicating well these days. Nonetheless, this is how my ideal endgame for show plays out inside of my head, and that's how it should be done imo.
What are your thoughts or particular ideas for the series finale, oh dear flist? How would it go and what would it contain, perfect ending or bittersweet? Satisfactory or open-ended? I am genuinely curious on what other people think.