Allies matrix has been released/leaked, don't know if I may link to it, but it seems Armies of the imperium is one faction which includes all SM, SW, IG/AM etc.
There are also 8 Xenos listed as their own factions, where only Eldar/DE and CSM/CD are battle brothers.
Tau has moved closer to chaos and necrons , no more taudar and no more traitor guard.
Nids still screwed by the matrix but anyone may ally with themselves now as battle brothers.