BAM two cents time.
Crazy predictions incoming.
So my guess is that we are not going to get any new model. I know Lady Aitia(SP) mentioned that we would get some kind of non-T'au thing, but rumors be rumors. We already got quite a few new models fairly recently. Models that kind of didn't really have to be upgraded, the T'au commander comes to mind.
Since GW loves to show off some kit bashed models, I bet we will see a section that has Gue'vesa in it. In fact, I almost guarantee it will be there. However, I doubt there will be rules for them, but who knows.
What I think they are going to do more than anything is restructure point costs. The Riptide is going to get a point reduction, and likely becoming more powerful in shooting. The Commander spam issue will likely be addressed.
Markerlights will likely be redone to some degree. The table itself seems to be unfinished, or half a table really. I would equate it to how most armies that had psykers only had three when they got their index. When the codex dropped it was bumped up to six. I bet we will see markerlights after 5.
More than anything though, I think "Sept Attributes" and stratagems are what we are going to get that will really be interesting. Other codex have a plethora of interesting and fluffy stratagems. Astra Militarum have a stratagem that costs 2-3 CPs and allows them to do an "artillery bombardment" after deployment and before the game begins. Each enemy unit on the battlefield roles a D6. They suffer 1 mortal wound on a 6. Not too powerful, but not too bad. Super fluffy and interesting. Other armies get things like getting to fight before they are removed from the fight phase as a casualty.
T'au will likely get some of our old formation rules baked in that will promote synergistic list building. Stealth suits might get the rules for Optimized Stealth Suit Cadre - Wall of Mirrors.
There might be a stratagem that lets the commander "blow up" when he loses his last wound, like the old signature system.
Stuff like that.
Sky IS Falling, T'au WILL Suck, Sell Me Your Models