Oh boy, I nearly started 40K with Tau before going for Tyranids instead. I really don't need yet another project, but if GW were trying really really REALLY hard to make me start them (or one of my projects evaporated in the night), here's what I'd say:
First off, no more absurdly large battlesuits. We've already got the Riptide and the Stormsurge, both of which are enormous, and those are just the plastic ones! If we were to get another BIG kit, I'd rather we got a plastic Orca (with better rules). It's a fantastic design, and IMO the non-Battlesuit Tau vehicles are majorly underrated, plus if they wanted to get more mileage out of it they could give it a gunship option.
That said I wouldn't mind more medium sized/light weight battlesuits, be they new designs or plastic renditions of FW models. The XV9 springs to mind with its awesome and Obari as all feth design, and likewise a "generic" XV22 HQ would be nice, as currently the only tabletop XV22 is Shadowsun.
New Kroot stuff is an obvious choice of course. I'd love to see some Vespid love too, for that matter.
More tanky goodness! Be it stuff on the Devilfish chassis or completely new. I'd personally like to see an artillery/heavy mortar equivalent (think the Wraith and its plasma mortar from Halo), and also I'd love to see the FW turrets make a return for the Hammerhead, though that's a pipe dream.
Finally, speaking of FW units getting Plastic Love (thank you Maria Takeuchi) I'd love the old drone turrets to make a comeback. They were far nicer and simpler in design than the plastic faction terrain we got- I think they'd be pretty good if you could actually buy them!