I would guess that I am not alone in this corner of the bloggyverse in how deeply I was influenced by the *original* Highlander movie (there can be only one, natch). The movie opened at the exact time that I was forming my high-school gaming group and it seemed to me that most of my conversations with those guys were peppered with direct quotes from the movie. Friends to this day, that is still the case. Not bad for a B-movie featuring a French actor playing a 400-year-old Scottish swordsman and a real Scottish cinema icon playing his ancient Egyptian-turned-Spaniard mentor.
So, quoting the movie aside, it is perhaps inevitable that Highlander-style Immortals would creep into my gaming. While still in high-school, I remember creating an Immortal hero for a friend's Gurps Supers game. When I moved to the DC-area, one of the other players did the exact same thing in a different Gurps Supers campaign (a character that would eventually end up beheaded). Another friend's AD&D 2e campaign allowed recently slain human characters a remote chance to rise again as Immortals, though it never happened while I was a part of the group. Finally, in my long running Gurps Voodoo/Vampire: The Masquerade mash-up, I had several of my players discover Highlander-style Immortality during extended preludes (I was all about secret mystical origins in the 90's). Even today, there are Immortals lurking in the background of my ongoing campaigns, though I've made some effort to tone done the connection to the movie mythology.
Why the fascination with Immortals? I mean, does it really matter if a character will live indefinitely beyond a campaign's limited time scope? Other than a few monster attacks or spell effects in AD&D, I've never heard of a campaign where characters aged more than a year or so.
For me, I am fascinated with Immortal characters for the simple reason that they have experienced so much history first hand. I love imagining how the weight of years might affect a character's personality. And I love exploring the price of immortality and dramatic situations that result when this price starts to seem too high. At first glance, immortality would be something that many folks would wish for without a second thought. Thinking through the ramifications of such a wish is something that never gets old. I guess that's why I was so smitten by Vampire: The Masquerade when it first appeared on the scene. At its best, that was a game that really milked the dramatic potential of the high cost of immortality.
Eventually, having Immortal characters with centuries in their past and potential ages in their future wasn't enough for me. I wanted to emulate that style of story telling that seemed to be a natural by-product of movies and TV shows about Immortals (and vampires). In the often excellent Highlander TV show(and similar shows featuring vampires), there was a kind of episode template where something that happens in the present reminds the viewpoint character of an event in the past. The episode flashes back to illuminate the back-story and there would be two or three more flashbacks before the final act, at which point the plot thread is resolved in a dramatic fashion in the present day.
I'm amazed that I still haven't attempted this. I've thought about for over a decade and I've always run up against the two obvious problems. First, there is the matter of handling the flashbacks in such a way that preserves the modern continuity without railroading the players. Second, there is matter of character experience and the fact that power levels are going to be drastically different in flashback. I never did solve these two problems to my satisfaction.
Now, I'm starting to think that I could pull it off. Even though it explicitly advises against immortal characters, Microscopehas demonstrated that these non-linear, time-jumping games are possible. The past is a mystery until you play it out and it doesn't have to be a railroad. Maintaining the present continuity doesn't seem like such a big deal any more. I can imagine that *anything* can happen to the Immortal characters short of continuity violations. And even then, some events that would seems to violate continuity could be explained in other ways (I'd leave that to the player character question to decide).
As for character level or point total... again, I'm not so concerned anymore. I think the baseline "level" for the group would be set in the present day. For flashbacks, characters already have plot immunity. There's less reason to worry so much about "balance" issues.
Anyway, I'm not running an Immortals game anytime soon due to current commitments. But I am going to give the idea more thought and hopefully I'll have some ideas that I can share.