Nagash is powerful enough that he could just dominate the undead. Cast necromancy powerful enough that he takes control of it all, forcing the vampires and liche priests to join himas his lieutenants or die.
Sadly no longer true, he would need to recover both his crown and hand for that, so its doubtful he could control all undead. On the other hand we know that Nagash has been gathering the most powerful Necromancers for a long time and he has some allies/servants within the Tomb Kings so he can still draw subjects from all spheres of the Undead.
I can't see him being brought back as a character tbh... regardless of the rumours... Would they release the rules and a model for Sigmar? No. For the Emperor in 40K? No. These figures work far better as bigger players in the fluff.
The principle differences here being firstly Nagash is still alive in the current setting (though this means little if u you at vampires current special characters) and more importantly he has had a model and rules before so it wouldn't be breaking any new ground in that sense.
Nagash is to me what the Crimson King was before he suffered character decay in The Dark Tower series, all evil, super mysterious power pulling the strings... he leads Kemmler to Krell and across the wastes, now he appears to be manipulating Mannfred to his will... MANNFRED, possibly THE baddest mofo in the world of men...
Put simply the easiest way for them to do this would be for Mannfred to have the Carstein Ring in his possession, which has been hinted about since 4th edition. The Carstein Ring was forged by Nagash as the one ring to rule them all (seriously lol

) and bound vampires to his cause. There are of course other reasons why Mannfred would willingly join Nagash: You have his knowledge of course, then you have the fact your either with him or against him (especially being a Vampire), the 1000 thrones prophecy of course, then you have the subject of the Vampires Curses which were placed by Nagash and he still has one (supposedly the most deadly) yet to inflict which he will of course know the cures for.
Regardless of how they do it, the rumours state pre-orders will be up on the last weekend of August so we will soon know!! Personally I am thrilled, he is the one model I have been hoping would be brought back into the game, as much for Thematic Army building as anything else. I hope the campaign book comes with a unique Army list for his forces that will contain Khemri, Vampire and Nagash specific stuff rather than invalidating or joining anything we have previously seen but I have seen no news whatsoever upon that side so that's purely wish listing.
Anyway the only concern I have is that it will be limited edition as I wont have the cash (though I can if needs must borrow it off my better half)...
crimsonsun