How would I have ended it differently... an excellent question, and one I hadn't put too much thought into other than, "Oh god not like this."
Ok, ask and I'll try to deliver. Well, not really, I just took an Ambien and started on a glass of wine. This may be terrible, but hopefully it won't be any worse than King's ending.
Roland and company are resting up near the tower, preparing for tomorrow morning: their long final walk into the unknown. Roland awakes before the others, and decides either one of two things: "If they follow me, they will die - so I will go on alone without them." Not a very Roland-like thought, so "They'll be in my way. This is MY quest, and I'll finish it on my own." The party is unaware of Flagg and Mordreds location and status.
Regardless of reason "why", Roland abandons his party and proceeds on alone. When morning comes, the party discovers he's gone and decides to chase after him. They leave, and soon Mordred and his new pet, a very confused/lobotomized Flagg, catch up.
Now what to do with our characters... We can play some deus ex machina and have an older Jacky Sawyer (and maybe a few wolves) show up and brutally dispose of Flagg and Mordred, and "save" the others, perhaps scattering them across the territories as sorta "knight errants" (keeping Jake and Oy together of course). Or, we can just kill them. No Jacky Sawyer crossover, but at least they get to die with their boots on. Not a little death either, a big long "this fight gets it's own chapter" type of battle. Alternatively, since our villains and heroes are all still more or less intact, spin off a separate book about just them and whatever their disposition turns out to be. If we let them die, they need to die honorable and valiant deaths.
Now we have Roland marching on to his tower, minus his horn.He gains entry and slaughters (not erases) the Crimson King in some fashion, with some great Client Eastwood style one-liners. He looks around, and his horn is there on a pedestal. He proceeds to the balcony, blows it, and cries out the names of all of those he left behind. Now, he marches on up to the top. At every room he learns some new truth, and usually a terrible one that he would rather not know. And when he reaches the pinnacle?
The man in black fled across the desert, and the gunslinger followed.