Personally, most of my stories start when I see a plot point I like in a show, say "I can do better than that," and start writing.
I sometimes get that thought too, and then I proceed to make up the better version in my head, and then I completely forget all about it, because it's just not enough of an incentive to write. Though I do appreciate the fact that others write "Like X, but better!" types of stories, since that means that I can ignore the "X" and go straight to "better". (When reading.)
edited 16th Jan '12 8:47:38 AM by fanty