I have built a world in which to write my fantasy book, and I'm currently writing my first book about it. I have it all planned out and am in the process of writing it. I have plans for a further book that develops naturally from this one, and will end with most things wrapped up quite nicely.
I originally wanted it all as one, until I realized the book would be far too long. So book 2 is essentially the second half of the story that begins in book 1 (neither would be complete without the other).
The second book, however, is not yet fully planned out. I have a lot of ideas, the main direction of the plot, character development and themes, but there are still big chunks of the story missing.
Having spent a lot of time building this fantasy world, I have lots of ideas about what is in it, and have (hopefully) constructed quite an in-depth world.
However, some of the ideas that I have wanted to put into the second book could potentially be expanded upon in much more detail.
I have no plans for writing any further books after the second, but I have had a couple of ideas for what one might be about, whether it is a sequel with the current characters or a spin-off with new characters in this fantasy world.
My problem is this:
Should I use up good ideas within a book that I'm already planning on writing that wouldn't be utilized to their full effectiveness, or save those ideas to use them fully in a book I may never write?
I realize that I am probably getting way ahead of myself in planning book 3 when I haven't even finished book 1, but depending on how book 2 will develop might affect what occurs in book 1 to potentially set up what will happen.
If there are any techniques for reusing ideas/places/scenarios without it becoming boring for the reader they would also be welcomed, but are not necessary. My main question is about best practice for utilizing ideas.