Personally, I’d – sort of – forget you’re writing a series. Pop that fact into the back of your head. Of course, you’ll still know you are but don’t let it be the focus of this current work. This is your first serious attempt at a novel, why give yourself unnecessary pressure? (See my thinking?).
As you plot your story, sketch your way through the first draft, you’ll find yourself scribbling side notes for future stories in the same series anyway. You don’t need to plan them at this stage, if your initial idea is strong enough, your characters are going to damn well demand more books as a matter of course.
That’s what I mean about forgetting about it, it’ll happen naturally.
Now, I may not be the most conventional of writers, so feel free to ignore my advice. But, the first book in the series should be the origin story? Hell. no. That’s tomfoolery to me. The story starts at the beginning, naturlich, but that’s the beginning that is right for the book you’re writing, for the story you and your characters are telling. Now, if you had already thought that that book would be the origin of Magic Eye, then fair enough. If not, then where you wanted to start would probably be the natural place to start. Listen to your gut.
I may be wrong, but I don’t think The X-Men started with X-Men Origins, or whatever.
I want to keep early back story for future use. I want the option to have a stand alone special story linked to my book that maybe I’ll flog to a magazine. I want to retain the option of spin-offs for future use etc.
Anyhow, not sure if any of that helps, or even makes sense, but good luck and Bon Voyage.