I had this exact idea and wrote it down but never sent it. Now I have owned your version for awhile and I just love it and how you executed this book. You can really tell how valuable this book is to GM's as every review is 5 out of 5 stars except 1 review dragging down the median.
I echo the sentiments of my fellow reviewers when I ask to see a second Monster Codex and here is a list that might be good candidates. Love to hear what others ideas might be.
Other possibilities are Cloud & Storm Giants, also Suli is a real cool playable race that could be added.
Finally I would also love to see a second Rival Guide but hopefully there would be more low level groups. It could be called Rival Codex and could also include a more extensive section for groups like primitive tribes, street gangs, crazed zealots, and evil monks than what is in the GMG.