(or "Different Singer, Same Song")
For those of you who want to know exactly how important a storytelling engine is to sustaining an open-ended series, you need look no further than Doctor Strange. Specifically, you need look no further than 'The Essential Doctor Strange, Volume Three', but let's take a step back first and look at the beginnings of the character.
Doctor Strange, like Spider-Man, is a creation of the Stan Lee/Steve Ditko collaboration of the 1960s, and like Spider-Man, they hit this one out of the park. The origin is perfect (Stephen Strange is an arrogant, selfish-but-brilliant surgeon who suffers nerve damage in an accident. He goes to a mysterious Tibetan mystic called "The Ancient One" for help restoring his hands, but instead of finding a cure, he finds a calling as a defender of the human race.) The set-up is wonderful; Strange lives in a mysterious, creepy mansion in Greenwich Village, with the Ancient One acting as his mentor (as a wizened old man in early stories, and later as a disembodied ghost, a la Obi-Wan Kenobi.) He defends mankind against the shadowy, oppressive forces that dwarf our comprehension, existing in realms outside of our own. (A Lovecraft-inspired touch that Roy Thomas dwelt heavily on during his many classic runs on the series.) Admittedly, Doctor Strange runs into the same problem Green Lantern has; at times, it's difficult to figure out exactly what he can and can't do with his magical powers. But the tone of the stories remains true to Lee and Ditko's (mostly Ditko's) conception of him as the lone guardian, the sentinel that stands watch over the human race and protects it from forces that could utterly destroy it.
Now, let's look at 'Volume Three'. Steve Englehart is writing at this point, and having just come off a couple of mind-blowing stories (one where Strange witnesses the beginning of the universe, and another where he recreates the Earth in exact detail after its total destruction), he decides to come up with another wowser. This time, it's a time-travel story, in which Doctor Strange meets a variety of historical figures like Sir Francis Bacon and Ben Franklin, examines their supposed interests in the occult, and learns "the secret occult history of America"...all the while, fending off a mysterious sorcerer known as Stygro.
Something about all that must have tripped somebody's "controversial storyline idea" alert, because Englehart's off the book two issues into the storyline, leaving Marv Wolfman to step in. Now Wolfman has to finish the story, but he has to finish it in a way that doesn't use Englehart's idea. This is where the storytelling engine shines. He might not be able to use the specific ideas Englehart would have used, but he knows the type of villain Doctor Strange faces, and the sorts of stories usually told in Doctor Strange. This allows him to plug in the mysterious "Quadriverse" as the villains behind Stygro, and although the story takes a different direction, it continues to work like a Doctor Strange story.
And then, after a few more parts, Wolfman leaves, forcing Jim Starlin to step in and finish the deal. But again, Starlin knows what a Doctor Strange story looks like. He understands the storytelling engine of Doctor Strange, and although he uses his own "pet" cosmic being, the In-Betweener, instead of Eternity, it's still 'Doctor Strange meets massive cosmic forces as the representative of humanity'. The same story goes through three writers, but they all are working from the same status quo, which allows them to salvage a workable tale from what could have been an utter disaster.