We have been wondering now for quite awhile what is going on with the idea of a Allied Supplement, or Mini-dex that has been rumored in the past. There are many indicators or little hints that this might be on its way (gretchin, genestealer cults, and more), but we really have nothing solid.
Here Shaso Iceborn touches on that release, and I am not sure it looks good for it. He then goes over and into a brief overlay of some Tau bits. I am hoping that we can discover some more information about what is happening with this supplement in the near future. (or more info saying its going away)
Please remember that these are rumors, and like always require a little bit of salt.
via shaso_iceborn over on Warseer
The mini ally dex has to my understanding been incorporated into the codex at the beginning of the book and will no longer be a separate compendium.
There is a Kroot HQ, but not in the conventional sense and he cannot command "Tau" forces (unless they bring back Prok but I have heard nothing on him)
Farsight will have a new model, it looks similar to the gamesday one.
Rail Rifles, Rail Guns, and Rail Cannons are the way they are distinguishing different rail type weapons in the new codex.
There will be 2 "new" special characters, one we may have seen before. (Aun'shi I think but do not have confirmation)