The Tau, of course.
From what I've been reading so far the Tau come across as nothing special. Pretty daring to defy the Imperium in such a direct manner, and their will to fight strong, spurred on by their foolish ideas of a 'Greater Good'. But This is nothing that the Imperium hasn't dealt with before. Many similar empires, with similar advancements to their much vaulted technology were crushed during the Great Crusades.
But who really hates the Tau? There are parts of the Inquisition that despise anyone loyal to the ruinous powers and a good number of Ordo Xeno's who hate all that Tyranids are. But no ones seems to dedicate themselves to destoying the Tau.
But shouldn't the Adeptus Mechanicus be persuing these blue fools? These young upstarts defy the Omnisah by creating Artificial Intellegence's. This naive race could doom both themselves and Imperium with their dabbling. At anypoint the Tau may constuct something beyond their control, some Intelegence that could turn their weapons against them and all races of the galaxy.
I'm surprised this hasn't come up yet. Of course, in this game there's little contact with the shadowy Heretek pursuing forces of the Ad Mech but the Ad Mech would surely be using it's influence with the Tech Marines, Certain Chapters and members of Ordo Xeno's to bring about the destruction of the Tau.