We know how (and where) franchises go to die, but what turns a one-off movie or a standard TV series into a franchise that can go on, sometimes even for generations?
After reading this post by commenter lightninglouie on all the different ways a franchise could go down (whether simply by reaching its natural end or by shooting itself in the foot), a discussion began about just what it took to get to the point of becoming a franchise in the first place. And the answers covered everything from the role of world-building to the role of a terrific ensemble cast:
I agree with most of your points with the exception of 5. There are a few ways of continuing it after most of the main cast leaves and still fresh and good if not better. Not many I'll grant you but there are.
Doctor Who, both the classic and nuWho basically do this every 3-4 years or so and not only has that become a staple of that show, it has become a massive event when they pass the torch at least in the UK.
Blake 7 was good at this as well to lesser degree and many Shonen Jump comics will replace characters or emphasize some more as time goes on. Dragon Ball is a great example, at the start of the Pilaf saga, the main characters are Goku, Bulma, Yamcha, Puar, Oolong and Roshi. But the end of the Kid Buu saga the main characters are Goku, Vegeta, Gohan, Trunks, Goten and Mr Satan.
X-Files is a special case because it really was about the characters. You could no more do X-Files without Mulder and Scully than you could do Sherlock without Holmes and Watson. They were the show, in a way that isn't true of the cast of a typical prime time drama or police procedural. The more Carter and the showrunners tried to expand the cast and focus on the aliens, the weaker the show got.
Doctor Who is really more about the character as idea than a role. That's why I mentioned the show under 5. As long as the writers and lead performer are true to the spirit of the show, it doesn't matter who's working on it. In some ways that's also true of long-running ensemble shows. Carter tried to turn X-Files into an ensemble, and it didn't work.
I think its all a question of world building... is your world interesting enough to keep setting films/books/comics in, and will it survive what your plot does to it and stay interesting.
LOTR, for instance, would be hard to go back and make a sequel to, because the ending of LOTR fundamentally changes the world, not so much because of the death of Sauron, but because the elves leave, the wizards leave, the magic itself seems to die. Then you just have ... some vaguely medieval place. But, say, the Harry Potter universe is still largely intact as an interesting, magical place. You could set new characters in a slightly different setting there and it could still be fun... you just couldn't lean on the Dark Lord Prophecy stuff as much.
What do you think? What particular brand of alchemy goes into making you want to revisit a fictional world again and again — and what makes you all too ready to leave it? Tell us, with examples in the comments.