We already know what the Allegiances are.
Nagash, Mortarchs, Soulblight and Death.
Skeletons and Ghosts don't need their own battletomes. I assume Deathrattle's will be standard units in all Mortarch and Nagash armies, while Nighthaunt will be synergize best by the Sacrament or Night allegiance.
It's far better for Allegiances to focus on more thematic things like Nagash or the Mortarchs. Rather then Army of Skeletons, Army of Ghosts.