Hypothetical Reader: "The series is called Isaac's Story: London. But I've read some of it now and nobody in it is called Isaac. Huh?"
Me: Isaac shows up in book 6.
Hypothetical Reader: "But I only bought the thing because I wanted some hot Isaac-based action."
Me: You don't know who Isaac is.
HR: "That's my kink, reading about guys called Isaac getting into near-future thriller scenarios. This is false advertising."
Me: Just imagine the protagonist is called Isaac, if that's your thing. Also, that's a really specific kink. There are a limited number of options where that specific set of circumstances is going to come up.
HR: "You'd be surprised."
Me: I've just imagined another page of conversation with a hypothetical reader I'm making up right now. Like, as I'm typing. And it's given me an idea for a story, which I now have to outline.
Me: I'm just saying, I'm pretty messed up. You'd be surprised at what surprises me.
HR: "Answer me, writer-man! Where's Isaac?"
So, where's Isaac?
When I came up with Isaac's story it was set a lot further into the future than the current series. 250 to 300 years, instead of 20. I sketched out five books, each self-contained but leading to the next, with recurring characters and an obvious series arc. The series is called Isaac's Story: Hong Kong, and I've outlined and part-written the first in that series (the title is Dark Age, for now).
But I needed to know what the near future would be like to understand the world. I needed to know what led to Isaac's story. So I sketched out the future, doing the necessary research and fleshing out how I saw the next few decades going, and enjoyed every moment of it. I fell in love with this world I'd built and I knew I'd have to write some stories there, if only to get to feel it, to breathe the air of this future London and watch the people go about their lives.
So I wrote something (The Bridge) which let me play in my new sandbox. And then I wrote something else (The Algorithm), and I set it 5 years later. The city was different, colder and darker. I had a new series, with an arc that followed the growth and change of a city, rather than a single person. I had recurring characters that I wanted to play with and observe, and I knew where the world was going already. Thanks to that original idea I already knew the ending.
Isaac's Story: London is a prequel to a series that hasn't been written. It's a look into the foundations of a world that you haven't read yet. That you can't (as of this post) read, because it's mostly in my head. I wrote it because I felt like it, and because I could. Thanks to doing this all myself, nobody could stop me.
Was it a good idea though, writing a prequel to something that doesn't exist yet?
That depends on what I'm trying to achieve. I'm in this for the jollies. I write because I have things in my head that want to come out, and it's either this or talking to the cat. People don't take you seriously if you tell long, complicated, action-packed stories to cats, so instead I've decided to write it down. (To be honest, people don't really take it seriously when you write it down either)
So yes, it was a good idea, because the books are good and I enjoyed writing them. One day the series will be a prequel to something that exists and the name will make sense. As I don't have a million readers hanging on my every word I think this is the perfect time to do silly stuff like this anyway.
So there, that's where Isaac is. He's in the future, waiting for me to finish with the backstory that led to his birth.
Me: No, because he hasn't been born yet.
HR: "But I want some Isaac."
Me: It's getting kind of weird that this conversation is still happening. You don't exist, and I've got better things to do than speak to a voice in my head.
HR: "I do exist. Once you imagined me I became a presence in the great unconscious cloud. I am an idea, and ideas never truly die, even if they never truly lived."
Me: This is where I start with the cat waxing, right?
HR: "That cat lives here too. As a metaphor it gets to live in the heights of the unconscious cloud, lording it over mere hypotheticals like me. Oh, to one day work my way up to Metaphor."
Me: Now I'm thinking of a story based around this brain-fart, and I need to get back to work.
HR: "If you write the story, and put me in it, will I then get to see Isaac?
Me: If you're an idea from my mind, and so is Isaac, then he should already be there.
HR: "You're overthinking this."
Me: That's what I was trying to say! Bugger, now I'm getting into an argument with... whatever you are.
HR: "I'm a manifestation of your---"
Me: Shhh. They don't need to know.
Me again: You're just trying to keep me talking, aren't you? To keep you alive.
Me again: HR, are you there?
Me again: ...