I had assumed that the Empire was next out of the traps, and there is still some scepticism about the VC being next up. There are certainly armies that need a more urgent update, wood elves, Bretonnians etc, but the vc DO need a new book for sure. With the boretastic mono build dominating the scene (with the interesting recent black knight bus causing some ripples, thankfully) the army has quickly grown stale in 8th ed. I'm a lifelong die-hard undead player, and even I have grown weary of the vc as they currently stand. Sure, you can make any list you want, but if you want to be commanding anything apart from bone-meal after turn three, you gotta use some variety of 'the list'.
There was some discussion on episode 34 of Garagehammer, with both Dan Heelan and Wayne Kemp making guest appearances. The consensus there towards an imminent vc book was one of cautious scepticism.
However, assuming the vc book IS next, what kind of goodies are we looking at? There have been some more common suggestions.
2. New plastic dual kit. Black coach and mysterious new unit. Presumably some other type of carriage/chariot. We already have a corpse cart, so this would need to be pretty different than another dead guy in a cart.
3. New plastic black knights. This is well overdue, so I hope this pans out. There are rumours of a new mounted unit, possibly mounted skeletons.
4. A return to the bloodlines. This would be great, as the current vc book is very Von Carstein orientated. I think this one is more a wishlist than a strong rumour, however.
5. Not new zombies. This hasn't popped up anywhere. Why the hell not? The clown-hand zombies will always have a place in my heart, but it's time for a new kit.
There are no real rule-change rumours as yet, just mild muttering about a major nerf to both vanhels and invocation, as well as a possible adjustment to skeleton warriors. I wont comment as of yet, as these are still low level rumours.
With the recent Terrorgheist, wraith and banshee additions, it seems odd that a new book is so close, but hey, I'll take it. More rumours as the come in, here on the mumblings.