One thing I can see happening is this.
The imperium of man is desparate because the golden throne is falling apart and they have no way to fix it. There are two races in the Galaxy that have the technological know-how to examine, diagnose, and fix the problem. Tau and Necrons. Obviously the Necrons are not going to be inclined to help Humanity stay alive, and the Imperium would have no reason to trust them. So that leaves the Tau.
I know it would be extremely taboo for a xenos to enter the presence of the Emperor, but I believe that in desperation they would allow a group of Tau to save the emperor. Now, after the fact would Mankind keep whatever promises it made to the Tau? Who knows. I think that the Tau are the only Xenos race that Humans can have a successful alliance with for these reasons.
1. Tau have honor. The Eldar also have great nobility, but they are dying out and they have little to gain by joining Humans. The Imperium already leaves the Craftworlds alone, so it wouldn't be that helpful. They both already have a "cease fire" if you want to call it that, and they have very similar enemies.
2. Tau have ingrained in their minds the greater good. If the Tau joined the Imperium they would work with the humans as long as the humans worked with them.
3. Technology. One word would be enough for this point but I figured I would elaborate. The Imperium doesn't know how it's machines work. They could gain much by regaining that knowledge. The Tau on the other hand could learn more of the mass production that the Imperium conducts.
That's my 2 cents. I'm off now.