Have you had enough of your speculative element yet? Well, today is another day to nail down the rules for how your magic, technology, or supernaturalism work in your story.
You might wonder why I'm asking you to spend so much time on this. Well, if you look at the most common weaknesses in fantasy and science fiction, you'll find that people dislike unbelievability. Now, a spec fiction audience will believe a lot. They're willing to believe in dragons, elves, magic, faster-than-light travel, ansibles, warp drive, trasporters, holodecks, teleportation, ghosts, werewolves, vampires, and even God, for the purposes of enjoying the story.
But as soon as the rules are broken-- as soon as people stop acting like people, or the warp drive only works when the hero needs to shorten the time spent travelling to the Bad Guy's hideout, but fails once he needs to make a getaway-- as soon as something inconsistent happens in your story, you lose your reader. They'll stop reading, or at the very least they'll stop believing, and that is death to your story.
What are the limits of your speculative element? What's the trade-off for using it? Magic usually comes at a cost-- what is that cost? If there's no cost, then what's the trade-off? What keeps it from being used all the time, for everything, or is it used that much after all? What keeps people from spending every hour in the holodeck? Why would anyone bother to learn to pilot a ship when there are transporters? Can your vampires go outside in daylight, or are they strictly limited to the dark?
Write down your rules, specifically focusing on what's impossible, and what should be established as unstable early, so when it fails conveniently in your novel, it won't be out of place or throw your reader out of the story. Establish the limits and boundaries of your spec element today.