I think there are three kinds of endings a story can have. The first is a Resolution where all questions are answered and nothing more needs to be discussed as the hero meets or fails to achieve their goal. This is the world of standalone; a world which I admittedly do not understand having been breastfeed on television shows from shortly after birth.
The second kind of ending is a cliffhanger. A cliffhanger is an unresolved Story Question. Cliffhangers often get vitriol from readers because it leaves them dangling from a cliff with their only option to what for the author to continue the story.
But here's the thing we all know; cliffhangers sell books.
Personally, I think there's a way to do a cliffhanger that doesn't get so much book tossing and angry reviews. That answer is the Open Door.
An Open Door is a new Story Question after the old one was answered. This method of closure invites the reader back on this crazy roller coaster ride of a tale. Going through the full spectrum of feels is what they came to you for in the first place. Books are a dopamine hit and authors are drug dealers. But we deal in the good stuff that goes straight to the heart.
The Open Door is a bridge to a new story by introducing a new complication without much of an explanation. With an Open Door you get closure because one storyline ends and another, which overlaps it, begins. I believe this method works because the reader or viewer gets that sense of closed satisfaction they crave. But before you fade to black or get to the last page, you wrench the door back open and introduce a new obstacle with that new story question from the same protagonist or a new hero.
Note: if it's a new hero, it works best to have introduced the reader to that hero during the course of the story as a sidekick or in a subplot.
So what's the next story you want to tell? In this Open Door scene, give a glimpse of the protagonist's goal and story questions. Yes, this method does work best if your stories are in a series.
I plan to write short story series romance, so that ending will definitely be an Open Door to other characters.