I think - and yeah, kinda hope - that they are buffing the armies that already have had a battletome, but had it before the GHB was released (and thus have some disadvantages because they rely heavily on rules that were changed afterwards by the GHB such as summoning), up to the power level / complexity level that they have established with the armies that had their battletomes released after the GHB.
So now after the Stormcast and Khorne Demons are done I assume (and yes, hope, since I play Lizards) we will see Seraphon, Skaven and perhaps Fyreslayers pretty quickly. And then IMO the other factions should get some love, especially those that don't have a battletome at all. I think Ironjawz, Sylvaneth and later won't get new ones since those releases came short-ish before the release of the GHB so they probably had those rules in mind already, and they have all the fancy stuff (allegiance abilities, artefacts, special spells and so on) already. They are also established on the new power level already IMO (debatable for Ironjawz I guess, but for Sylvaneth I'd say they are).
As for Death: I wonder if it would be better to have a Deathrattle and Zombie Horde and Tomb Kings Battletome seperately or a huge cool one for the whole grand alliance... I think the former plus a few models to make each Death army competitive on its own would be preferable. But again, that's a personal opinion, I'd like to start a pure Deathrattle army with tons of Skeletons but they seem quite weak to me.