As far as I can see, there is really only one missing class: a psionic one.
The idea of players playing monsters as PCs has been around since day 1 with Gygax saying you could play a dragon if you want, as long as you start at level 1. Then in 2e they had a "Complete Book of" supplement that outlined how to play monsters as races. Not a new thing. But it's so rare that I don't think they need to put out a book with all those monsters listed as races. Volo's guide already covered that topic on how to do it. So I don't expect to see much more.