While perusing the Doctor Who news, I came upon this article that tells us that the character of Captain Jack Harkness, introduced in Season One and returning for the last few episodes of Season Three (not to mention the spin-off series Torchwood, which I've never seen, and don't especially expect to) will be back for some episodes of Season Four next year.
That got me thinking about Brigadier Lethbridge-Stewart, who had the distinction of showing up in Doctor Who as a supporting character (pretty much never the main "companion") off-and-on throughout the span of the original series, beginning in the era of the Second Doctor (played by Patrick Troughton) and continuing all the way through to an appearance in the very last season of the original series (Season Twenty-Six) alongside the Seventh Doctor (played by Sylvester McCoy). In fact, if you add in non-televised and other-character appearances, actor Nicholas Courtney (who played the Brigadier) has appeared alongside every Doctor from the First through the Eighth, the only actor with this distinction!
While it would be tons of fun to see the Brigadier to show up alongside David Tennant (the current Doctor: the Tenth) in the new series (and Nicholas Courtney's still around, making such theoretically possible), it's clear that the actor can't keep going on forever, and so it makes sense for the producers behind the new show to introduce their own elements. It seems to me that Captain Jack is being set-up to fill the Brigadier's role: a military-trained friend of the Doctor's able to deal with alien threats on Earth when the Doctor may not himself be available. Where the Brigadier had UNIT (short for United Nations Intelligence Taskforce, which itself has shown up a couple of times on the new series), Captain Jack has Torchwood (and indeed, Torchwood appears destined to fill in the "UNIT" role in the new series, since apparently the real United Nations isn't too keen on it's nominal involvement. At least, that's the rumor I heard some time back). Where the Brigadier's character took a back seat to more explicit "companions" such as Liz Shaw and Sarah Jane Smith, Captain Jack has played second-fiddle to Rose Tyler and Martha Jones.
But like the Brigadier, who kept showing up again and again for years after the departure of other companions (and other Doctors!), Captain Jack (who himself has already outlived one Doctor, and is getting to know his second) seems poised to outlast the companions he was introduced alongside. Will fans of Doctor Who ten years from now be talking about Captain Jack the same way as current long-time fans talk about the Brigadier? Only time will tell.