Within the context of this document, I'm defining a long-running series as either a television show that contains more than one season, a book or movie series that contains more than three installments, or a narrative franchise. The important distinction here is that the series is running long enough to no longer cleanly fit into a three-act storyline. Trilogies are being ruled out because each book of a trilogy can be loosely divided into a three act structure. When more than three books are involved, such a division can no longer be cleanly employed.
For a narrative to have significant longevity and endurance, it needs a good framework for each of it's installments.
A narrative framework faces a similar need for balance that a standardized protocol faces. If a narrative framework is too narrow and specific, its installment will be formulaic and shallow (It won't be able to support a variety of stories). If the framework is too thin and lacks constraint, each installment will have to reinvent the wheel and the narrative will struggle to maintain both continuity and consistency (The thinner the framework, the closer the series is to having no framework).
At least for now it will be useful to examine some examples of long-running series
In general, series make for better franchise origins than one-off installments. In other words, radio serials, television shows, and comic books series are a better medium to start a franchise than a book or movie.
The Star Wars franchise has a shabby narrative framework. It was never designed to be a narrative franchise. It became a merchandise franchise before it became a movie franchise.
The first Star Wars movie has a well constructed story with a variety of quality parts that all fit together in a neat package. However, it largely relied on one-off mechanisms that don’t support much variation.
As a result, most Star Wars movies either have little of the qualities of the original movie, or are a rehash of the first movie.
The main exception is The Empire Strikes Back, That movie is an oddity not only within the Star Wars franchise but also beyond it. The Empire Strikes Back has a really weak and off-beat foundation, yet it works far better than it should due to excellent ideas and execution. It uses a recipe where the sum is normally less than its parts, but such impressive duct taping was employed to remedy the recipe that the parts manage to mostly work cohesively.
The second Star Wars trilogy did establish a better narrative framework than the first trilogy, but it is a very framework that outputs very bland stories.
The Mandalorian was an innovative break-out hit. At first glance it might appear to have breathed fresh life into the Star Wars universe.
However, it is a short breath. Like the first Star Wars movie, the Mandalorian relies on one-off mechanics. It does little to establish a more general story writing framework.
Star Trek has a solid, well-designed narrative foundation. This foundation is derived from a rich heritage of previous successful serials in other genres, particularly westerns.
Some of the constraints of its foundation are: