The Ending:
Lorcan and Sam leave Dean to die in a fire at the visitors center at Medicine Wheel, and he is rescued by Castiel, who takes him to the Fishers’ home. This place is a mystical in-between sort of place, made up of gardens (like the Chalice Well gardens in Glastonbury), with lots of wells and a World Tree. Dean recovers, and he and Castiel confess their love to each other. (Sex may possibly occur here. I’m seeing a long conversation before or after about why it matters so much, that the earth is preserved.)
D: “I’ve known it for years. Known it in my heart. My dad told me just before he died and I’ve always denied it, but I can’t anymore. If I can’t save Sam, I’ll have to kill him. There’s nothing else to do.”
C: *is quiet a moment, then kisses him* “No, you won’t.”
When Castiel is away Maya looks after Dean, and tells him he may stay there and rest as long as he needs: Castiel is preparing has been ordered to go after Lorcan and Sam. “No one expects you to do what must be done,” she tells him; and what must be done is Sam must die. He is the final seal, and if he uses the Cup at the site of the new Grail Castle at midnight on Good Friday, the door will open and Lucifer will walk free.
Castiel must kill Sam and will bring the cup back to the Fishers: “It’s what we should have done in the first place. We will keep it safe here. It won’t quite be on Earth, and Earth will suffer for it, but it’s just not safe there anymore.”
Dean, of course, is furious: Sam is all he has, and if Castiel kills him Dean will never forgive him. And if they’re just going to keep the cup in this otherworld, what was the point of all the quests?
Maya tells him there is one way for him to get help in finding a way to save Sam, and takes him out to the gardens to the World Tree. “What do I have to do? Climb it? Meditate under it?” “Hang from it.”
She hangs him by the ankle from one of the branches, and while he’s hanging he hallucinates Jo, Chelsea and Anna (maybe even his parents, too?), and the normal lives he could have; then Castiel comes and cuts him down, and Dean realizes as they argue that love is his weapon and that’s how he will save Sam. (“I love Sam more than anyone. Even more than you. That’s always been my weakness. It’s also my strength.”)
They drive to SF through the desert, and Castiel makes the car go so fast people only see it out of the corners of their eyes. (Include a little tidbit [and there should be more of these throughout the story, urban legends being born, etc.] about people telling tales of a phantom black car that looks like it’s on its way to Hell.)
When they reach SF and Grace Cathedral, it’s Good Friday, and it’s raining. They go into the Cathedral to see if Sam is there, but services are going on and they can’t look for him. They realize that they have to go through the outdoor labyrinth to reach the Grail castle, and that they’re unable to walk straight across the labyrinth and have to go through the path.
But first they have to face Lilith, who is waiting for them in the labyrinth (or on the grounds, I’m not sure exactly) with the minotaur. (And possibly Sam and Lorcan, captured: Lorcan ranting about how she broke her promise. I’d actually kind of like to see her tell him he’ll live forever, he’ll just be doing it in Hell.)
Dean sends Castiel to protect the people in the church (Castiel with a flaming sword that doesn’t go out in the rain) and goes into the labyrinth, where he uses the sword he got at one of the Grail challenges to fight the minotaur. He’s about to slay the beast when he looks into its eyes and realizes it’s a slave, and cuts off the collar Lilith has been using to control it, thus setting it free. It snatches up Lilith in its jaws and runs away. (Another tidbit section: people call into the police to say they’ve seen a bull escaped from the zoo, but no wild bull is found. Some people also report the bull was carrying a child, but these reports are dismissed. Second tidbit: “In hell, it is whispered, in its twistiest darkest caves, sometimes you can hear the hooves of a great beast and a little girl screaming. But of course this is just a story.”)
(Maybe end all the little tidbits with “But of course this is just a story”, like the “That is another story for another time” like in The Neverending Story. Arc words! Hee.)
With Lilith out of the way, Sam, his eyes golden, appears to walk the labyrinth and open the door. Lorcan demands to know where Lilith is: he’s earned his reward and wants eternal life. Dean tells Sam he loves him, that he’ll always be his little brother, but also that he will kill him if he has to. “The world can’t fall because of you.”
OR, Lilith and Lorcan are there at the same time, which probably makes more sense, and the breaking of the final seal involves Sam’s blood in the Grail, thus desecrating it. Oo. Making it into the opposite of what it is: instead of a door connecting heaven to earth it’ll become a door connecting hell to earth, thus freeing Lucifer. So Dean has to rescue Sam, banish Lilith, defeat Lorcan . . . and then Sam says, “Hey, guess what? I’m evil anyway!” and then all the other stuff happens.
When Dean, Sam, Lorcan and Castiel reaches the center of the labyrinth the area changes: walls comes up, hiding the city and the hedges and the cathedral, and the Fishers appear. They ask Lorcan who the Grail serves, and he says, “It serves me.” (Maybe he tries to fill it with blood from Sam first?) This is the wrong answer, of course, and the Grail (or the Fishers) destroy him.
Dean picks up the Grail and the Fishers ask him the question, “Whom does the Grail serve?” He says, “It serves everyone who asks,” and it fills with sweet water, appearing as a richly jeweled and gold vessel.
OR, first he heals Sam with the question, along the lines of “Why do you suffer when there’s healing right here?” and has him drink, and then drinks and heals himself.
The Fishers tell him to drink and heal himself, and he does so, finally healing the wound in his leg the demon gave him in ch. 1. Sam weeps, and Dean tells him he loves him as he gives Sam a drink of the water. The Fishers tell Sam he is forgiven and the Grail will purify him of the demon blood. Each woman gives the men a kiss and a blessing, that they have served the Grail well and it will be safe in the cathedral for years to come.
*Dean saves Sam with 1)love 2)his name 3)the Grail itself–calling Sam by his name reminds Sam of who he is, and loves gives the Grail its power to heal–love for family, one of the stronger types.
The castle and the Fishers disappear, and they see the services have ended in the cathedral. They go inside and hide the Grail and the other treasures inside, the Grail once again a plain clay cup; it will be hiding in plain sight as one of the cathedral’s Mass cups. “Who would think to look for the Holy Grail in an Episcopalian church? Nobody, that’s who.”
When this is attended to, Uriel somebody else, maybe Michael or Gabriel or Raphael, who will have to show up earlier in the story for this to make sense most likely Zachariah since he’s canon, is waiting for them in the nave to take Castiel back to Heaven. “You disobeyed.” Dean tries to argue with him, but Castiel tells him he’ll accept whatever punishment he’s given, and they kiss and part.
Or, Castiel just goes back, knowing he’ll be punished, without another angel having to tell him why or that he needs to. He’s always known he’d go home when the job was done. And now I’ve foreshadowed that Castiel knows this, so he just won’t be surprised when Zachariah shows up.
The Castiel ending
*Then all the Castiel-climax stuff I’ve already figured out 
.
*Still want Castiel to “turn human” at the end? or skip that and keep him an angel? I don’t want to imply at any point that he’s being punished for loving Dean or for using his body for more than being a messenger.
*In the end, after Lilith is defeated and the grail is safe, Castiel leaves to be disciplined and is stoic about the possibility that he’ll be destroyed. A few weeks later he returns, mortal: his sentence is to live, grow old, and die; and however he lives out the rest of his life will determine what happens to him next, just like everyone else. Dean, of course, is happy to see him.
*The final confrontation is either on Good Friday, like the Perceval stories, or on Easter Sunday. If it’s on Good Friday, Castiel could come back on Easter–or Dean expects him to come back on Easter and is disappointed when he doesn’t (and then will show up later instead. Midsummer’s Day, for instance.) Dean could think Castiel will be back on Easter and insist on waiting for him, but when he doesn’t return they leave SF and go to Bobby’s for a bit, and then back on the road . . . and then Castiel comes back.
S: So I’m going to have to put up with both of you now. Fun times.
D: Suck up and deal, bitch.
S: Jerk.
They both look at Castiel, who is just smiling
C: You two fight it out. I’m driving.
*I’m not sure at this point if I want it to be the body we’re familiar with or a different body that Castiel has when he returns.
*start the epilogue/denoument chapter with a note that few hunters know this last part of the story
*Sam has Dean take him to a monastery in Santa Clara, where he stays until June to recenter himself. Dean’s on his own for a few months, and notices there’s a lot less demonic activity than he expected–but there are more sightings of fantastical creatures. Bobby tells him he’s seen dryads, for instance.
*Remember to foreshadow the hanging man with Dean.
*I want Bobby to be involved more, but I’m not sure how at this point.
*As they drive out of SF to their next destination, the boys notice how green and healthy the land is looking now. Spring has finally come.
Something about how hunters don’t know this part of the story: “In many ways, the story ends there. The grail was safe, the world was still neither heaven nor hell, and the two sons of John Winchester were back on the road. but in many ways there is one more tale to tell.” And then the last chapter.