Allavandrel said:
One thing that bothers me about the bestiary theory is that the text doesn't make any references to greenskins. To me this strenghtens the proposal of a Tome of Corruption kind-of-supplement, and it may indirectly suggests that there will be a greenskin sourcebook...
The reference to 'a night that would not end' leads thoughts to the situation in the Empire in IC 2522 during the Storm of Chaos - maybe a scenario is included in the fourth book.
Possibly you're right. Keep in mind, though, that they didn't mention a LOT of other beasts too. I would be surprised if FFG ever puts out a single-beast/monster book given the tie in with cards and components. You're more likely to get a broader swath of coverage in each book, though with something like a dedicated bestiary you might see a lot more content delivered on each included entry.
And to be honest, single-beast/monster stuff doesn't typically sell as well as the blended stuff that more people want and need.