I think a good ending should resolve the main conflict in the book somehow. For a plot-centered book, it would be like destroying the Death Star or restoring the king to the throne. Once that big "event" is completed, have the celebration and end the story. Like in Star Wars original trilogy, we don't need to know if Han and Leia get together or what Luke decides to do.
In character-driven stories, the most satisfying ending should either be Happily Ever After, Happy for Now, or "Character Has Grown and is in a Position to Move on".
Serial fiction has its own rules I'm still trying to figure out for endings. But if you're trying to write an ongoing piece of fiction, you need to time your endings to fit with your larger main series idea. An example of this would be having your main couple not get together officially until several books into the series instead of that being the first completed arc of your series.
For an "emergency ending" - you might need to rethink if you are a book series or a serial. With a book series, you have the freedom to plan everything out. You ultimately have unlimited time to write your work and put out the best possible story. If you're writing something more serialized, you run into more emergency situations. In that case, write the ending like the season finale of your favorite TV show. When a season comes to an end, the showrunners might have no idea if they'll get another season. So they write the finale to be a big in case that is their show's permanent ending. (idk if this helps any)
