There's no such thing as not good enough. That's one of the things that's so great about this website. You don't have to be able to write a full story, you just have to be able to put down in writing what you want to see happen next.
A51, while certainly a very capable writer, proves this: a large quantity of his posts are simply one or two sentences with the most obvious continuation of the previous one. Sometimes doing that can remind or inspire someone, and the branch grows because of those one or two sentences. And if that doesn't happen? No harm, no foul.
Literally the worst thing that can come of you trying to write a new branch in the thread is that someone else makes a parallel branch and it grows more. And as far as you're concerned, that's still a good thing, isn't it?
So don't get yourself down. You've proven with even this post that you have what it takes to make a contribution to YAWYW. If you know a direction you'd like the story to go, write it. Then you'll be able to say that you are a part of one of your favorite stories.
