What I would love to see for the Tau, even if they don't get their own codex, is something Kroot centric.
With they way codices have been going in 9th, Kroot will likely not get the <Core> Keyword which means they will loss some of the few benefits they actually can still get (like the Ethereal Aura and buffs). Kroot are already barely scraping by as it is, and with the chance of new models being very small, there are a few things that could be done with what they have.
1. Bring back the Knarlocs: FW made these before, it's not strange to think they could bring this line back. Even if it's not in plastic, just having them available to purchase and no longer being Legendary would be huge. If they stay roughly as strong as they are now, the Knarloc riders are a solid melee option and they Greater Knarloc is very strong. I don't think Tau themselves should have much melee, but the Kroot can and should fill that role for them.
2. Kroot HQ: Make Shapers HQ units, OR even better, make a "new" unit that uses the same profile as the Shaper with some stat/ability changes to be an HQ unit. No new model needed, though GW could do that and also milk this with a limited edition promo model; they love that kind of thing, and for something like this, yeah I'd buy it.
3. Improve Kroot utility: Fix the wording on the Hidden Hunters stratagem to make it good. If we get a new HQ, some kind of new buff from them. Add a few new stratagems, at least one melee focused one for Carnivores. Make a "Sept" for the Kroot that doesn't effect Tau units just like the Septs don't effect Kroot.
3.5 Food Buffs: Each Kroot unit can select a pregame buff based on the food they most commonly consume and are "Shaping" into. This could be done for "free" buy giving them a general points hike, or better yet, keep the points low and make these optional for point increases. Since GW like's their D6's for randomization, I'll list 6 potential options. 1) Psyker Powers: The unit can manifest / deny 1. It knows Smite and a custom Kroot one, likely a stealthy/defense buff (though I would love a whole Kroot Psychic table). 2) Ork Ferocity: +1 A and +1 Advance/Charge. 3) Astartes Resilience: +1 T, 6++ FNP. 4) Human Tenacity: +1 Ld, +1 BS if remained stationary. 5) Eldar Alacrity: +1 M, +1 WS. 6) Beastial Nature: +1M, +1 S, +1 T, +1 A, +1 W. This unit must always move at maximum speed to the nearest enemy and must attempt to charge it if possible. This unit may not shoot.
Lore wise, Kroot don't feed on Tau, Necrons (they only made that mistake once), Tyranids, or Chaos.
4. Krootsade Support: Kind of like what I said in 3.5, there is so much potential customizability with the Kroot and if we get it nowhere else, we should at least get it in Crusade.