I have a story. This is not the same thing as saying, "I have an idea." In my particular case, I've been searching for this story for a while because numerous people had asked me to write it. "Write a sequel to Cold Words," they said. Easier said than done.
My linguistics stories are like puzzles, with lots of complicated interlocking parts, so as a result I feel I have to know where they are going or I can't even start. "Cold Words" was like this. The idea of writing a sequel had several advantages over starting from scratch:
1. I wouldn't have to design a world with aliens.
2. I wouldn't need an entirely new main character.
3. I wouldn't need an entirely new alien language and an entirely new voice.
All of this is an incredible time-saver. But as many who have tried to write sequels probably know, there are also some serious disadvantages.
1. Any subject of a major revelation in story one has to be a starting premise for story two. You need to inform readers of the "secret" (or former secret) in case they haven't read the prequel, but it can't be the point, or the story will just be covering the same ground.
2. Having a world and a character, and even quite a clear sense of chronology and what would have happened after the first story ended, does not mean you have a story.
This number 2 can effectively stop me in my tracks, because until I have a story, I won't start writing (this is not the case for all writers!). I might also remark, of course, that the lack of a story hasn't stopped many sequel-makers, at least in the case of certain movies I have watched.
What is a story? I'll sum it up this way:
Character X discovers Y and must accomplish Z before A happens or else B.
This rather closely resembles the elevator pitch for a novel. It distills story essence down to its bare bones.
In my case, I've been gathering elements that I like - Rulii and his dependency on humans, his relationship with Parker - and questions that people have asked me - about gender relations among the Aurrel, etc - and trying to fit them together. At last I've gotten somewhere. I don't want to spoil any surprises, so I'll just say that Character X is still Rulii, Y involves Parker, Z involves Parker and an Aurrel Female, and B has to do with human presence on Aurru. It's always good to ask yourself, "What's the worst thing that could happen to my character now?" and try to have it stand in that B position. That makes for a much more exciting story.
In fact, it's good to ask whether you have a story even if it's not a sequel, though sequels are probably more prone to the lack-of-story problem. Giving your character goals and stakes makes everything more exciting.