One of the interesting things in the new book, is that yes, Mannfred is working to get nagash to return, but they don't give us any insight into why? This is a big change as far as Mannfred goes from when he first appeared in the 4.ed Undead book, and it also goes against "all" reason that GW has provided on Vampires so far (again, necrarchs being the exception).
Here is a quote from the 4.ed Undead book regarding Mannfred and his thoughts on the return of Nagash:
"An ancient familiar had brought word from the south that Nagash stirred once more, reaching out from his ancient fortress to bend the dark to his will. That thought worried even Von Carstein, for who knew what that ancient, evil liche was capable of. Once he had come close to mastering the world. He might yet if he gathered his power to him. The thought occurred to the Vampire Count that his resurrection and the awakening of Nagash was connected but he thrust the idea aside. He was his own master, and he had his own plans, and even now they approached fulfillment"...
How did it go from that, to Mannfred becoming a Nagash "lackey", with little or no explanation? Thats a pretty significant change, and GW could have given us some insight as to why the big change I think. It could be that he was not prepared for the dangerous knowledge in the liber noctis though. That is a possiblility. For those whom have read the Vampire Counts triology, you might remember that he initially didn't succumb to the temptations of studying the book, as he feared it would instead become the master of him, and he had no intention of becoming anyone's slave..
Malisteen: It might be that they are waiting for more armies to be released for 8.ed first, but that is just speculation though. As far as the Storm of Chaos campaign went, it didn't change that much in the end, but there was quite a few novels released that revolved around it, so from a pure timeline basis, quite a lot happened in a relatively short period of history. I feel like 've been waiting for that Nagash summer campaign forever, and for a while I suspected that with the boost of undead corpses etc after the Storm of Chaos, Mannfred might suddenly launch his atack in the empire. Instead Mannfred just gathered corpses and returned home...