In general, it's a bad idea to write any sort of media tie-in novel on spec. Because everything needs to be approved by the licensor first.
Here's how it usually works: A publisher buys the rights to publish SPACE VIXENS novels, then goes looking for freelance writers to write them. No surprise, they usually go with people who have experience writing books based on similar movies and TV shows. It's just easier that way.
The prospective writer writes a proposal or outline which is submitted to the TV people for approval. Typically, there are various stages of revision until you reach a plot that everyone body agrees on. Then and only then,
does the writer start working on the book.
I don't wake up in the morning and think, "Hey, I'm going to write an 80,000-word WAREHOUSE 13 novel and then try to sell it." Instead I usually get a phone call from an editor I've known for years, asking me if I'm available to write a WH13 novel.
Ditto for Star Trek.
As with any career, the hard part is getting your foot in the door, but it helps to have some general experience in science writing and/or editing.
(I started out as a full-time science fiction editor at Tor Books, who had also written for various sf magazines and anthologies, including a handful of BATMAN stories, before John Ordover tapped me to do a DS9 book.)