There has been some speculation on this, and I'm getting chatter now that seems to reinforce a few things, so I thought I'd post what is floating my way.
Tau Codex early next year.
Kroot and other existing allies are there, however they are limited. There appears to be an "Allies" supplement which will be out about the same time and include many more options. (aside: Dare I start a Spring of Allies moniker...ah well then it will end up being 2 years from now... )
Crisis suits completely redone.
Stealth suits completely redone.
Pirahna has new option and I am not sure if it will be reboxed together or be sold as an upgrade kit.
New Pathfinders were seen being worked on a while back.
Ethereals expanded, new unit options around them.
There is a FW Tau model which has a unit entry in the codex. (My take is this might actually be a new plastic kit taking over a FW model, and before anyone yells GW doesnt cannibalize FW...yes, they do, they have, and they will.)
The floating chair ethereal was seen being worked on for a new model.
New fast skimmer sized between hammerhead and pirahna.
Flyer has dual gattling cannons
Flyer has option for rail guns
Flyer has bombs (not missiles!?)
Rule rumors (these are rumors people! I hate posting rule rumors cause I am not involved in that. Please take fwiw)
Railguns have a JOWW effect, they can hit multiple models in a line, but if hitting vehicles has a max number of hull points it can cause. the way this is described is it draws a line from the barrel to table edge, every model under starts taking hits starting with the closest. if it hits a vehicle or structure with HP, its strength drops, once it hits 3 or a table edge or fails to make a penetrating hit its done. rhino line's watch out!
There is some rumor of an Anti Psyker weapon. like night fighting, but no psychic powers for a game turn.
There is no flyer dedicated transport, and the existing flyer is going to be competing against some very good other options
The jetpack troops have some option to "pop up and shoot" from behind cover.
All I have for now