First off, the image above is a conversion. None of the rumors below are discussing a Tau Titan, but instead a Dreadknight sized suit that has the aesthetics of the image above.
That being said, a large Dreadknight suit sounds rather fun for Tau. You should heavily expect more and more people moving towards having a small collection of Tau in their armies, after all already a unit of Tau Broadsides with dual Railguns is a fantastic addition to any army list.
The following were taken from Warseer. The image above comes from doing the google image search "Fil Dunn Tau"
Anybody who might be interested in what tau models (or at least one of them) you can really expect next year might want to type "Fil Dunn tau" into a google image search.
Just to clear things up there IS something very similar to that image coming for tau.
via Stickmonkey (an old quote from a long time ago)
I do have sources telling me there is a bigger suit, which has been reported from others. Reports have it being a ranged beast(metaphorically, not literally) with 2 twin linked rail cannons and a missile option.
Basically according to my sources it's pretty much a Dreadknight sized suit with dual (note not twin linked) rail cannons and SMS with a rule for something I will call "hail-fire" in that it shoots massive barrage of weapondry if not moving in the movement phase.