Seeing as we've gotten Kharadron Overlords and Disciples of Tzeentch now this year, and we're going to get the Lumineth Realmlords and then Sons of Behemat around late spring/summer time, you can see there is at least one battletome so far for each Grand Alliance except Death so far. So it makes sense to balance it and give us a Death Battletome some point this year. There are a couple of options they could do for this:
New Faction: Probably got a 95 percent chance of not happening this year, because we got the Ossiarch Bonereapers last year plus the fact that the Lumineth Realmlords and Sons of Behemat probably are the full representation of this years new factions. However, a couple of the rumour engines do show some death looking stuff, so I guess something isn't totally impossible? However my bet is this won't be the case. If anything, seeing a Vampire tome if not Soulblight is probably the best thing they could give us.
Soulblight: Something we should have, at least some form of one tome for vampires. If they split Soulblight from the Legions of Nagash to get their own battletome, while keeping them in there similar to the Nighthaunt, then we could see a brand new great tome for Death. Give us a new vampire hero on foot, a unit with two different builds on foot, new Fell Bats and new Blood Knights, with endless spells and potentially a scenery piece if really wanted and boom, an army appears. Only a few kits, like 5 or so would be needed and a release like this around the later part of the year would be good. You could even release them around Halloween to make the theme of a vampire army just right.
Deathrattle: Much like what Soulblight could have as being a separate force but inside Legions of Nagash, Deathrattle is sadly a less likely option for a battletome. New skeleton beasts and heroes would've been great to see, but the Ossiarch Bonereapers have already taken that space and so a Deathrattle army of spooky scary skeletons is out of the questions for now most probably. Keeping them in the legions is fine, but their own tome would be unexpected and cool to see. But again, not at all likely.
Deadwalkers: Like Deathrattle, not likely. A zombie army full of animate corpses would be cool, with a zombie hero perhaps and several corpse units. Tying in the Necromancer and Deathmages in general with this army could be cooled thematically, focusing on reanimating the dead on full scale. But while this would be interesting, there may be a lot to have to add to make them their own faction separate from Legions of Nagash.
Legions of Nagash: Probably my bet for this to appear with a new book this year, potentially with the models I named in the Soulblight and other bits above, but other models I'd like to see redone would be Dire Wolves and a new Wight King on Skeletal Steed model, with a possibility for another hero on foot, like a zombie hero or something. Just generally a new remake of old models, similar to the Slaanesh release when a lot of the old models got updated for here. Legions of Nagash should also get some endless spells, and be the last of the current four armies to get their own, and a scenery piece could also be here like a graveyard or a catacomb.
At very least, if we do see only Legions of Nagash redone, let's hope we don't get the same scenario as the recent Seraphon had and none of the old models get updated or added to. As much as we could get a scenery piece, it's becoming a bit frequent in AOS armies so far and personally I'd like to not have too many more for armies. For beginning collectors you don't want to spend 35 pounds on a scenery piece to make your army work while for the same cash you could boost up your collection up with more units and heroes. Legions of Nagash should have model redoes and if they don't or if any of the armies that could split don't I'll be disappointed.