From lore point of view, yes, they do. But is Death really a GRAND alliance? I mean is combination of humans, elves, dwarfs, lizardmen equal to undead? Are they as powerful as Chaos? Don't think so. I think Nagash and Company didn't have so much impact in the End Times for example as those races mentioned earlier. Or did I get something wrong (know the story just from forums, not books actually)? It seems a different league to me. And that's one of the reasons they (death) get less attention.
Yeah, I'd played it a lot. The one thing that didn't match lorewise was IG/Ork decks.
So I expect that in time we will be having more Death factions but it still will be much, much less than other GAs have.