I’m doing another draft of Coldfire and have just come to the realization that an event I have planned later in the series is not hinted at in this book. Not even a passing mention. I plan to rectify this.
When it comes to writing a series where there is a definite end to the story, rather than a collection of standalone novels in the same universe, we need to foreshadow upcoming events. To foreshadow is to hint at what is to come so the readers are somewhat prepared, even if they don’t know what they’re being prepared for at the time. With the right amount of foreshadowing, even the most outlandish event can be made logical. In fact, foreshadowing is also used in standalone novels, but is especially important in a series.
In my mind for a later novel in the series I’m working on, the entire world is changed by a single event that also changes the characters’ agendas and personal situations to the point where everything is wildly different from the previous books. I can’t do this without dropping hints beforehand. The slow buildup to this event will (hopefully) be like a hidden thread travelling beneath the main plot of each novel. If I tried to do this any other way, I would be in deep trouble.
I’m planning an extra scene in Coldfire that will introduce the villains earlier and also give the main character a mystery to unravel that won’t be resolved in this particular novel, especially due to his cynicism on the subject.
J.K. Rowling is the master of foreshadowing. You could pick apart the Harry Potter books and find endless allusions to later events. The series is an excellent example of foreshadowing if you want to see it in action.