Author's Note: Since the DT cycle is on a repeating loop this could very well be canon eventually as the loop draws to a close. Please keep in mind that this is a treatment and not an actual story. In the words of my college creative writing professor "Concrete details are the foundation of good writing." That is why Stephen King is as good as he is and I'm not, but still I want to get this idea down and communicate it so here it is without further ado.
It all starts as Susannah is giving birth to Mordred in the Dixie Pig, with Walter/Flagg supervising. Mordred does what he does best and sucks the life out of his victims. Then Walter appears and tries conning Mordred into being his pawn and entering the DT for himself, Mordred sees through him and kills him of course. Or does he? I see Mordred as being the reincarnation of the CK, not exactly his twinner as Mordred has 4 parents (2 human, 2 demon) but as the CK 2.0. It all goes back to the gunfight at the Breaker's Prison in early Book 7 when the Beam is saved and the CK's attempt at actually toppling the Tower are ruined, without the Breakers he can do nothing. I believe this is what sent the CK over the edge and turned him from the godlike-entity of Insomnia and Blackhouse to a raving old man reduced to throwing grenades to Roland in the original finale.
Anyway the CK foresaw this descent into madness and impotence and downloaded his full consciousness through Mia into Mordred as he was being carried in Susannah. I suspect this happened sometime in Book 6 when Mia had possessed Susannah's body and took her to the Dixie Pig. So Mordred is really the reborn CK so now we don't feel bad when he casually casts aside Walter and uses his body for food.
Skip ahead to near the end of the book, Roland and Susannah are at the Dandelo's laughing themselves to death then instead of duex es machine Susannah realizes what is going and wounds herself mortally with either gun, knife or household device, her grimacing temporarily stops her laughter and she is able to kill Dandelo before Roland dies laughing, sadly she knows by saving him she will soon die herself. Roland comes to the grim realization that his ka-tet is down to him, he is again alone. That is until he finds Patrick Danville, in the Dandelo's house. They proceed through the fields of roses to the Dark Tower and find it soaring in black majesty above the sea of crimson. Then much to his surprise Roland finds himself in the midst of everyone (those whose names he shouts out at the end of DT 7 (and Brown, Zalton and Allie from Book 1) and with them is a total stranger. His face is painted greasy white, with funny fiery red hair and a costume that reminds him of a jester from Gilead. Roland does not now that this individual is actually Pennywise the Dancing Clown and is actually preying on Roland's fears. The false Jake approaches Roland and pulls the Maturin figure out of his backpack and presents it to Roland as a gift. Roland takes it and the next thing he knows he is holding a spider statue. He looks up to see that his friends have been replaced by the CK's many servants, a who's who of the Red. Walter as Flagg flanked by Lloyd Henreid and the Trashcan Man, Pennywise, Barlow, Atropos and the Overlook possessed Jack Torrance to name a few.
Roland grits his teeth and realizes that he is surrounded by many enemies, more than the 6 bullets of his gun could kill in one swoop and with one hand he could never reload in time before he and Patrick were mobbed and tore to pieces. Flagg steps forward with a demonic grin to deliver some exposition. "We are the red, the crimson" he gloats triumphantly, "You could say we are the kings of the universe of discord! No, not we, I, I am the King of the Red Discordia, All Hail the Crimson King!" Flagg then morphs into a were-spider and Roland can see with the naked eye the burgundy/crimson auras straight from Insomnia that emanate from the were-spider's body to the followers of the Red. Spider Flagg laughs and continues "or still saying we is still appropriate, we all make up part of the Red". Flagg proceeds to explain that the Crimson King is not a single person but in actuality a collective consciousness of all the entities of the Red, he makes a joke about democracy really being mob rule and how he/it will go to the top of the Tower and turn it in on itself destroying the Tower and all of reality just for the sake of pure nastiness, the fact the CK entities will be just as effected as everyone else is irrelevant, just as long as suffering increases. Hearing this Roland empties his revolver into Spider-Flagg/CK, to no avail; bullets are too weak for him. Laughingly he uses telekinesis to tear the gun out of Roland's hand and cast it beyond vision.
All is lost when a sudden hum and light appear behind Roland, emanating from Patrick Danville, who is no longer human but now possesses an angelic visage. As Roland's eyes recover from the light he sees the there is not just one angel but many; one to counter for each minion of the CK. Patrick stands in the center of the angel formation, the brightest and most majestic of them all. Suddenly Roland realizes it! Patrick is Gan! "Head for your destiny, the Tower my son," is the divine voice that Roland hears in his mind, he turns and practically vomits at the grotesque demonic form that coalesced from the CK and his minions to battle Gan. It is important to note hear that the emotional reaction, the hopelessness that Roland experienced during the Coda of DT 7 is conveyed here in the God/Devil battle that is about to begin. This is because the battle is that intense, Roland for everything he had seen and lived through, his survivors guilt were all still inadequate preparation for the cosmic battle of the ages is about to start before him. With a prayer of thanks on his lips Roland sprints towards his destiny, the Tower, making it to the entrance and stepping in.
Yes it is open-ended, the outcome of the Patrick/CK battle and the contents of the Tower are up to the readers imaginations.