As a media tie-in writer, I do not write fanfic.
Everything I write is edited.
Everything I write is vetted through the licensor and they have full authority to recommend (read: insist) on changes to ensure the license is held up to a certain standard.
Everything I write comes with a contract and I get advances/royalties on my work.
Everything I write appears in a bookstore via a legitimate publisher (Pocket Books, Berekley, Putnam-Penguin).
I am not a big-time author (yet - God willing), but I've written Star Trek, Doctor Who, Gauntlet Dark Fantasy, the Incredible Hulk and am working on a Stargate SG-1 novel.
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Now, in answer to your question regarding parody, the answer is . . . it depends.
Obviously there have been a number of parody books out there (Bored of the Rings and Star Wreck both come to mind . . . very funny books and I bought both at bookstores). Hell, Weird Al has made a career lampooning other people's stuff.
Ben Bella makes a living doing "unauthorized" books, but they do a pretty good job of minimizing anything "actionable" in their books. The Firefly and Battlestar Galactica books they did are pretty darn good. Also,the current trend of "The Philosophy of X" books are doing well and they are unauthorized as well.
The question becomes is a publisher willing to take a chance on some bean counter at Paramount or Viacom getting their underwear in a twist over what you've done?
If a story is strong enough that they think they can make enough money to cover any potential lawsuit, then yes, it is a marketable piece of work.