For some things, I leave myself options so that I can use characters or ideas later when I need them. Take MeLaan from Mistborn for instance. You can go and look back in the original trilogy and see her there as a minor character. At the time I thought, “I’m going to use her in the next series, for sure.”
Now what I usually do is when I’m starting a series, is write the first book in the seriesthen sit down, and say “Okay, what worked about that, what can I expand upon?” Then I outline the series with those characters and ideas. Once that’s done, I go back and revise the first book to match before I release. So I don’t know everything while writing the first book, but by the time I’m through the revisions I usually know what I should be foreshadowing.