That all said I want to expand and say I am not unkeen to the idea of drone tanks and an expansion on drones in general. Drones in the general culture are pretty popular, and I'd like to see the option to take an army that consists mostly of drones. Gun drone spam was a passtime in Dawn of War DC. I'm willing to bet they have some ideas in mind, I just can't think of any as I feel just about every field role they could play, they are playing except troops.
Hell, they should just take DX6 Remoras and mainstream them, which wouldn't be objectively complicated to do, but you have to break the weapons/hull into more parts to work on a typical sprue/injection mold process. Remoras are gorgeous in their own respect and are a nice side-play to Piranhas. They allow T'au to greatly expand their flyer capability without up-scaling to a Barracuda or multiple Shark fighter/bombers.
I'd like to see some piece of signature support that allows Deny the Witch. I don't think it will happen, despite fluff about Tau and their apparent inoculation from the Warp.
The Orca going mainstream would be the way you get a "helicopter" type transport for the T'au. Considering the Orca is OOP this isn't necessarily off the table but then again, how often did anyone read up on the Orca and think "every T'au army will need one of these." But yeah it could transport something like 6 crisis suits at a time. With the new manta strike rules though I've yet to see the major tactical advantage to justify going out and procuring one.
Like here's the scale, and here is the kit: as big as it is, those suits are a tight squeeze. Link: https://flic.kr/p/iUYCZ6
Vanguard Void Suits, the XV46, I'd like to see that, I just think Forgeworld will do it first.
Kroot/Vespid Characters would be an interesting add as well that wouldn't hurt anything.