When you are working on a topic you might not realize what the real subject-matter is. For example, in my just-begun project, I am beginning to realize the subject might be modernism in relation to nationalism. It might be "translation, parody, kitsch," as in AL.
Usually the hidden subject emerges late in the process of writing. Then the hidden subject can be the ostensible subject of the next project.
I now realize that my hesitancy about my 2nd Lorca book is due to the idea that it won't be as good as Apocryphal Lorca, the book that took my career to a new level... combined with my greater enthusiasm for the next two books in my Lorca Tetralogy. Of course one should always be more enthusiastic about the next project than the last one.
I almost never re-read anything I've written in the past unless I have to. When I do re-read myself, though, I realize that I always had it. It being ideas.