
I can't speak for every author, but I know when I start writing a series, I walk into it mostly focused on the series at the time. While you hope for a spin off series or two, because we're passionate about these stories, (why would we write them otherwise?), it hasn't happened for me yet.
Well, during my beta process of The Rule of the Damned, I realised there was a bit more context needed to the monster heritage I refer to in this series, and so the prologue "Freak Show" was born. But then writing that made me realise "The Originals" as my one beta dubbed them, needed their own story. The more I delved into this, I realised I needed a duology, because there was more than just one book in this story. Mostly, I was asking myself about their creation, and how they set up these lives in America, the one we read about in The Rule of the Damned.
So I plan to write this duology at some stage (don't ask me when because I don't know if I can write any faster!). Because of the potential spoiler territory it goes into, it's most likely going to have to wait until I'm done writing The Titan Syndicate Saga, but right now, I'm just taking this a book at a time.
But an origin story about Dracula, Van Helsing, Jekkyl and Hyde and Hecate? I can't speak for every dark fantasy lover either, but I know I would love to read an urban fantasy with characters like this. So, I'm going to write it. (My approach to most of my books at this point of my career.)