The Sunday magazine section of the New York Times features an important and exciting piece on good teaching: "Building a Better Teacher." Classroom practices and mastery of content knowledge as it relates specifically to teaching are the two foci of the piece. For my purposes, it is the content knowledge that need a response, although what is said about classroom practice is significant.
If we are going to move toward Flexbooks more, then mastery of content will become more and more important. It's one thing to download a textbook and start teaching with, another thing altogether to be able to find errors in it, weed out what is irrelevant for your specific class, and add what you think will stimulate your kids. Conversely, if teachers begin working with Flexbooks, and they begin to adapt, improvise, edit and correct, I would argue that they will become better teachers very fast. If they have the horsepower to begin to engage a text at that level, they will grow and exceed their current limitations.
As part of a group that worked over several years to write and publish a series of textbooks, I have personal experience of the way the process stimulates, renews, challenges and improves teachers and teaching.
So, in addition to the actions being taken in graduate schools of education to improve classroom practice and those intended to improve knowledge of content relevant to teaching, let's add another action step: show beginning teachers how to create their own material. Get them to be creative and original about the content they are teaching. Engage them with the curriculum and the tools. Talented teachers will rise to the task; mediocre teachers will play above their level; incompetence will become immediately apparent.