The more you write, the more ideas you'll get, so don't panic. I'll wrack my brain to plot out the book so I know what happens, and then end up adding a MAJOR plot point that will drastically alter or add to the rest of the book. I've done that for both books now. The first time was by adding Mrs. Moon as a central characters, the second was by giving Alicia's screen saver serious relevance. Neither one of these is planned, so I would tell writers, you have many more and much better ideas than you think, just let them come to you, you'll grow as you go, and realize at some point down the line that you're actually a genius. XD. Just don't fight your new ideas, it's too easy to try to make yourself stick to the plot points, but new concepts are your friend, if a scene looks like it's going to change itself, let it flow.
Mrs. Moon entered on this chapter and she never left XD:
And Alicia's screen saver is mentioned here: