Honestly speaking, I am writing serials. From the very beginning serials have always been how I planned out my stories more than novels. Since I always thought beyond just a single book, beyond just a trilogy. I merely thought about the story, where it begins and where it ends.
And here is where I will talk about what is a serial.
#1 Serials are not novels cut into small pieces
Serials in general, the format is very very different from novels. Novels must have a beginning, a middle and end by the end of the book. It might not apply to serials, where a volume of it might have very little to do with the central story.
Serials are more akin to TV shows, anime and manga. In the sense that they can go on for Seasons and years before reaching the climax. They don’t have to do so within a book.
However, it doesn’t mean that a serial doesn’t need progression. It still needs them, even though that is for another time.
A serial can have a large cast of characters, but most importantly the characters must be compelling. And the characters have to be effectively used in such a serial.
As serials is much more reliant on strong characterization, and character arcs and development as the plot is not nearly as tight as a novel.
A novel can rely on a simple idea, a concept. For a serial, you need more than that. You need to know the direction of the story in general. You need to have a good sense of where it is going, how to raise tension that is also relevant to the character.
And most importantly, there must be an end goal to them. Somewhere where the story can stop and will stop once it has progressed to that point, or in general it will feel directionless.