Ok here goes hard wishlisting
It has to be battletome not some random offshoot underworlds warband with couple models.
If I can't get my old school heavy armoured vamps I want at least option for many sub lists of different themes.
All plastic and different style models regardless of the theme they come up with at GW
Horrors of the night and halloween ghoulies, though this might be closer to Legions of Nagash style book...
Bring back all tiers of vampires from lowly thralls to full blown Vargheist style monster (centerpiece model if Nagash is not in this book)
Different kinds of vampires from weak melee but great spell casters to dragon slaying blood dragon melee monster
Mansion on a hill might be too much asked for terrain piece but something cool
Shyish style undead farmers, or living allies
Ruleswise: Hard hitting vampires with something to help big monsters to stay on table more than one round.
Something to make it interesting to play, from summoning undead to making baby vamps from enemy heroes, not just your basic run to melee army
Endless spell or warscroll ability batswarm, for either movement or prevent enemy shooting