Another longish rant, but this one on writing.
Ideally every story should be about the single most important event in the life of the protagonist.
Series stories need to be about the most important event in some character's life.
TV shows are frequently franchise shows: Comedies and cop shows (i.e., their mission is to set things right, be they cops, cowboys, doctors, lawyers, or starship captains)
Comedies are simultaneously the hardest and easiest type of story to write, their audiences simultaneously the most unforgiving yet forgiving.
All that matters in a comedy is "Did they laugh?" All other rules of writing, drama, art, and literature fall to this.
All comedies are franchises as well: How will these particular characters make me laugh? Good comedies invariably follow the laughs, bad comedies try to force them.
Back to the core idea of story: Each tale is about the single most important event in the protagonist's life.
An epic story requires an extensive background full of character and incident to fully explain the importance of the story in the character's life (viz. Dune; try boiling that down to a satisfactory 2,000 word short story).
Conversely, a short story like "The Gift Of The Magi" tells us everything we possibly need to know about the newlyweds; anything more would be superfluous and dilute the impact.
The problem with telling the most important thing in character's life is this:
Whaddya gonna do for an encore?
By nature, most stories are designed to be poor source material for sequels.
Which is not to say good sequels are impossible, just that they have to start with the assumption that the previous story was the most important thing in that character's life up to that point.
And it can work:
Nothing wrong with series films, but they aren't the same as a sequel.
As a rough rule of thumb, if you can enjoy a story without having to have experienced the previous story to understand it, it's a series story, not a sequel.
There are exceptions. Huckleberry Finn is a sequel of sorts to Tom Sawyer; it transcends its source novel and becomes one of the greatest works in American literature (the other Tom Sawyer sequels are tedious). Individual films or stories in a series can rise above their roots (though I'm unable to think of one that rises to Huck's level of brilliance); they tend to be remembered as really good examples of the series as opposed to stand alone works of merit (Goldfinger, f'r instance).
The single most difficult kind of story to write is a prequel.
Because by definition, a prequel consists of the stuff that wasn’t good enough for the original.
Now again, there are exceptions.
J.R.R. Tolkien had plotted out The Lord Of The Rings but decided to do The Hobbit as a shakedown cruise before starting on his epic. It really isn’t a genuine prequel since it was released well in advance of his epic trilogy.
Sometimes an original story gets adapted to another media, and when it does part of it has to be jettisoned for time / budget / aesthetic reasons. A prequel that incorporates such material usually is still superfluous.
Nevada Smith is a prequel to a gawdawful movie called The Carpetbaggers (based very loosely on Howard Hughes’ Hollywood career) which is based on an equally gawdawful Harold Robbins novel of the same name. In both versions there’s a cowboy actor named Nevada Smith (based very loosely on real life cowboy / soldier of fortune-cum-actor Tom Mix).
To pad his already turgid tome, Robbins threw in a novella-sized flashback to Smith’s pre-Hollywood life, filled with copious Wild West violence. Joseph E. Levine, The Carpetbaggers’ producer, was savvy to recognize this material would make a serviceable Western & cast Steve McQueen in the role (Alan Ladd played him in the original film).
Mario Puzo’s novel The Godfather has numerous flashbacks and references to Don Vito Corleone’s boyhood, his fleeing Sicily for America, his early introduction to crime, and his skills as a gang leader is bringing together several independent criminals to form the core of his “family”.
Francis Ford Coppola’s film of The Godfather keep the story rooted in the late 1940s, eliminating the flashback / background material. When that film’s success demanded a sequel, Coppola & Puzo turned The Godfather II into that rarest of films, a prequel / sequel by using Don Vito’s story as a counterpoint to Michael Corleone’s consolidation of power.
All of this is a really roundabout way of saying I’m really looking forward to Ridley Scott’s Prometheus since he’s finally going to reveal who / what the Space Jockey was and why he / she / it was tootlin’ ‘round the galaxy with a starship fulla Alien eggs.
 It may come as a shock to some that I’ve never seen any of the Potter films & only read one book in the series (#3 or #4, IIRC). The book was well written, I can easily understand why so many people love the series, but it just didn’t connect with me. As someone once observed re Edgar Rice Burroughs’ Tarzan, “If you don’t discover him by age thirteen, you’ll never discover him.” My knowledge of the books is based on what I’ve gleaned through cultural osmosis & from what friends whose opinions I trust have related to me.
 This guy is wrong-wrong-wrong-wrong!