What are the limitations, things your characters can't do? Details sell sci-fi — your research will pay off in time. Decide: The history of its worlds if your novel is about a new version of our own world, then figure out how this new version came to be.
And they are super visible about it, because they live online. Think about some of your favorite science fiction and fantasy stories.
If humans have evolved to breathe underwater in Chapter 1, your heroine can't drown in a swimming pool in Chapter 3. Change an existing law of the universe. Or countless classic SF writers, for that matter.