now that tome of decay is coming out (after it has sufficiently rotted) what do you think they will do (if anything) about plague zombies.

You could probably make your own with the minion rules if you wanted too, or perhaps they will be in there somewhere, after all the Tome of Decay would be the right place for them.

yeah I will probably use melee horde minions for them or something

While not as iconic as plague marines they will probably be discussed in the Tome of Decay, hopefully with the plague ogryn as well. With luck they will bounce off of the Horde Minion talent in Tome of Excess and give a bonus for acquiring a hoard of plague zombies.

maybe they will be part of the nurgle special minions