It's Day 28 of the 30 Day Doctor Who meme for me, and the subject it asks me to talk about - a Doctor Who episode idea I created myself - is one I actually find difficult to approach. That's odd, perhaps, given my forays into writing fan fiction, but it's true. Still, I'll take a stab at it.
Day 28 - An Episode Idea You Created Yourself
I would argue (in a spectacular burst of understatement) that fan fiction differs in many ways from the writing involved in its sources, and serves a different need for fans. In the case of DW, the original source is largely episodic television writing. The differences are, therefore, two-fold. Not only is the content different - fan fiction most often explores specifically what original books/movies/episodes don't - but the goals are different and the very format is different. Put those things together, and the gap between fanfic and source DW material becomes really wide.
I am currently trying to put together a story which will (I hope) eventually become part of a virtual DW season, and I'm working through those differences. I'm realizing that most episodes include some mystery that has to be solved, to a greater or lesser extent. And writing anything with a mystery requires a lot of discipline. For one thing, you need to know all the answers before you start writing, because you have a specific time limit, or space limit in which to work - you can't just wander through an unfolding plot until you discover the answers to the questions you might inadvertently ask (which is how I normally write, god help me.)
The plot I've developed is surprisingly mundane (perhaps I should say "traditional"), in that it focuses on the disappearance of a major character, and the questions of a) just who among a given number of additional characters are responsible for the disappearance and b) just why the bad guy or guys has done so, and c) just how. It's all very plot-oriented, without a lot of the focus on characterization that I normally am most comfortable with. It's a challenge that I'm enjoying, though.
That's really dancing all around the actual subject, I suppose. When it comes to a specific story idea? Anything I could think of has probably been considered, assayed or completed by other writers ... but after a little thought, I think I'd like to do a "bottle" episode, one that involves staying in the TARDIS and exploring Her near-infinite depths. I really want to see companions being drawn into the undiscovered country inside our favorite dimensionally transcendent blue box.