How do you feel about sequels and series in general? Do you write them and or read them? Do you know ahead of time that you will be writing a bunch of stories or do you make that decision later on?
As a reader, I tend to like sequels/series better with published fiction than fanfic, but it all depends on the story and how much thought the author put into the decision to turn it into a series. J.K. Rowling, for example, always knew that the Harry Potter series was going to be seven novels long, and although each one tells its own story, they go together to tell a bigger story. You can see evidence of her thorough planning in the way that it's structured.
When I decide whether or not to read a sequel of a story I liked, there are two questions I ask myself:
1. Do I care about these characters enough to want to read more of their story?
2. Is there really more story to tell?
If I'm left hanging after the first story and do care about the characters, I'll almost always read the sequel. If the first story stands alone and wraps up nicely, I really need to be able to answer "yes" to both of those questions. I think one of the mistakes authors make with sequels is writing them for no reason other than that they want to or because they can't let go of their characters. But sometimes there just isn't another story to tell that's going to be interesting without going over the top. There have been several sequels to stories I love that I haven't bothered to read or started to read and stopped because of just that.
As a writer, I wrote way more sequels in my first two years of writing fanfic than I have in the last decade. Some of those series really could have just been combined into one story, and some of them have sequels that were completely unnecessary and over the top. I learned that just because I have an idea for a sequel doesn't mean I should use it!
My only notable sequel in the last ten years has been By My Side, which is the sequel to Broken. When I was writing Broken, I did not intend to write a sequel to it, and so it wraps up with a nice, neat ending and can stand alone as a story about Nick being knocked down by an illness and finding a way to get back on his feet (or foot LOL). That said, I saw the potential to further explore the romantic relationship he forms, which was really just a subplot in Broken, so I decided to write a sequel that continued his and Claire's story with their relationship as the main storyline and the aftermath of going through cancer as more of a subplot. Was it necessary? No, but I do think there was definitely more story to tell there, so in my mind, it was worth writing, and I think the readers who cared about those two characters and their relationship in the first story found it worth reading.