Ghassan_
No, mine wasn't a series.
Honestly, I don't even know how it could go that long. I had a basic plot and framework but obviously, it grew beyond that.
In my case, the readers got invested in the characters as well. They'd give their opinions and thoughts, and some even asked for a few things. I actually gave a 'reward' to the ones who commented the most i.e. I asked them if there was anything in particular that they wanted to see in the novel.
Hence, two of the main arcs in my novel were based on the reader's request. Some of the mini-stories were also based on the reader's requests. I always told them, if it's within the plot of the novel, I will put it in.
Another way I expanded on it was when there was a question or query about someone/something that happened, so I would then incorporate those questions to be answered in the novel itself.
Being a slice-of-life type of story, they grew with the characters in the novel. They were quite invested in it and somehow, there was always something to explain, something to show, etc. So from the original 80 chapters, it became 900 chapters ... cough.
Yes, I'm still rather in a daze about that.