There was once a Tau thread discussing all the various elements of the Tau empire in which there were a few mentions of Astartes that willingly joined the legions of the greater good. I propose a thread wherein we discuss further the possibilities of such a faction.
I had made mention of the general qualities of the Astartes serving under the Tau. Their leader is old. Really old. Not old enough to have met a primarch or anything, but old enough, like the vast majority of their numbers to become tired of it all. Tired of the constant service to an emperor who hears not their prayers, tired of the unending centuries of war, wrapping up one campaign against a faceless, nameless enemy, only to be shipped to another planet, another war, another exterminatus.
He had grown weary some time ago, and when his ship was destroyed on the way to yet another battle, he awakened from his Sus-An sleep to find himself amongst none but the dead. Knowing not who had attacked, or how, he knew only that his brothers were dead all around him, the ship devoid of power, his hope-devoid heart accepting reality, that here, in the black of the abyss, was where it ended. And then the ship arrived.