There are several bundles on the GW website when it comes to Tau, and often most of these are looked at as potentials for new formations for book releases. With a campaign book release coming up for pre-orders, some of these formations could easily just be that book only. However, a couple of the new bundles specifically say that they are for codex: Tau Empire. Only two of the bundles mention this.
So we know at least two of these will be formations in the codex.
Here is a link to check out the Tau Bundles
via a reader on Faeit 212
I don't know if you've noticed, but on the GW website, under the tau empire
collections section it has a couple of formations - the Heavy Retribution
Cadre and the Optimised Stealth Cadre. When you look at the descriptions
for these it says "This bundle gives you everything you need in order to
field the Optimised Stealth Cadre, as found in Codex: Tau Empire." - guess
these are two of the formations that will be in the new codex then, and
confirmation that a new codex is definitely coming.