(or "There Won't Be Only One")
'Highlander' is a relative rarity among storytelling engines, because it didn't start out as one. In fact, writer Gregory Widen turned in a script that seems to defy sequelization at all--it's the story of the final battle between a group of immortals that have lived in secret among the human race, summed up by the iconic line, "There can be only one!" (As an aside, it's important to note the difference between coming up with a sequel and coming up with a storytelling engine. Coming up with a sequel means finding the logical extension of a stand-alone story, while coming up with a storytelling engine involves setting up a premise that can generate multiple stories. It's the difference between 'Die Hard 2' and 'Dawn of the Dead'.)
But as the movie developed from a minor flop into a slow-burn cult hit, it became evident that "only one" wasn't enough. The movie developed a franchise that wanted more, and wouldn't take, "Everyone's dead except Connor!" for an answer. About the first attempts to develop the story into a storytelling engine, the film's theatrical sequels, well...the less said the better. They were slapdash, didn't fit the lyrical fantasy tone of the original, and they were stuck trying to tie into the ending of a film that was fairly definitive in establishing itself as the conclusion to the whole concept.
The TV series, though, took a different tack. TV writers are, by definition, good at creating storytelling engines, because they're all too aware of a) the challenges of writing twenty-two episodes of television on an enormously tight schedule, and b) the need to write five or six seasons in order to get the lucrative rewards of perpetual syndication (and DVD release, nowadays.) So they took the risky step of breaking down the 'Highlander' film to its component elements, finding those that would support a storytelling engine, and discarding the rest.
The first big one they let go was the protagonist. Connor McLeod was a spiky, alienated immortal burned out on caring about the mortals who lived and died all around him. His story was all about finding his ability to love again...and that's a wonderful plot for a one-time movie, but as open-ended TV series go, it doesn't work. (Plus, actor Christopher Lambert probably wasn't willing to commit to a weekly series.) So Connor gained a "cousin", Duncan, who was a bit more genial, connected to the world, and generally audience-friendly. (It says a lot, by the way, that despite only appearing together twice in the franchise, Lambert and Adrian Paul have a chemistry that easily convinces you that they spent lots of time together off-screen.)
The second thing that had to go was the Gathering. Again, this was just necessary to the development of a storytelling engine as opposed to a one-time story. The Gathering, and the final battle of all the immortals, is by definition the end of the story. (Unless you suddenly decide they're all aliens from the planet Geist or something.) Setting it in an "alternate universe" (the official explanation, never actually referred to onscreen) where immortals are more prevalent, and new ones appear all the time, gives the writers the chance to tell more stories than Widen's conception allowed. (The movie occurred in the series continuity, by the way, but Kurgan was just another nasty immortal dispatched by Connor, not the second-to-last of his kind.)
Which brings up another point; the series' mythos had to be widened to accomodate more stories. Some recurring villains had to show up, simply because it's hard to create a new villain every week (as touched on in the column on the Punisher); the organization of the Watchers was created to help create supporting allies and enemies, and various recurring foes like the Four Horsemen, Kalas, Ahriman, and Xavier St. Cloud helped keep the pressure off the writers. And of course, supporting characters, both mortal and immortal, generated stories of their own--Richie, Amanda, Joe, Tessa, the list goes on and on. (Mainly because the series wasn't afraid to kill off supporting characters.) Indeed, by Season Six, Duncan was barely featured in the series at all. (Primarily because Adrian Paul was already thinking of moving on.)
The revamped Highlander concept ran six series and spawned a further two movies (and several books and comics), a respectable run for any series but all the more impressive for a series that started with the end of the story. It's surprising, really, that it took until 2006 for the comic book to arrive, since a comic can conquer the last hurdle that the series had to being a true open-ended storytelling engine...comic book characters, unlike actors, never age.