Looks like the preorder teasers are up. 3++ has more screen shots.
Things of note. They confirm that it was chaos that kills the ethereals, and that farsight went rogue discovering Chaos exists.
New wargear like the Earth pilot array for the riptide.
New characters like the return of Brightsword
(though this is someone taking up the mantle, and using fusion instead plasma)
“The Tau can outshoot pretty much any other force, but rather than talk
about the tactics of a tau gun line, this supplement will focus on how
to get the most out of an army comprised entirely of super mobile battle suits.”
Finally crisis suits as troops perhaps? As long as there aren't horrible restrictions such as last codex, this should be good. Anyone else looking forward to this? I'll have to finish up my Farsight model.