As the editor of a first year composition open textbook series, I can tell you that the composition textbook is not the best choice here for making the author's pointes. It's actually hard to create a single textbook that serves every class/teacher/institution. This is why there are so many different flavors of composition textbooks offered by commercial publishers. That is also why the open textbook project I work with, <a href="http://writingspaces.org/">Writing Spaces</a>, is producing a series of books with essays on writing, written by teachers for students, so that they can be mixed and matched for the specific classroom context.
That being said, first year composition is the major textbook market in the US; more students have to take it (or at least exempt it) than any other course in higher education. It would be an excellent starting point for a national major open textbook initiative in terms of creating resources that would be most widely used.