I'd like to see some things like templates for skeletons/zombies rather than have them waste space with a huge number of iterations. I'm also assuming that as a FR supplement it'll be expanding into yet un-updated monsters of the realms, which will be good as well.
I'm fairly easy to please when it comes to bestiaries, so at this point it's more of what I hope doesn't make it more than what does. I hope we don't get another giant swathe of content for giants and dragons, for example, with demons/devils trailing not far behind that.
I will echo the sentiment that I'd like to see some more fey, it's always been really weird to me how often they get the short end of the stick on content.
Oddly enough, if you think about it, there are very few monsters that are really FR-specific. Kobold Avenger mentioned the three most iconic ones, and beyond those there isn't much else besides a handful of monsters like the malaugrym. I'm paging through the 3e
Monsters of Faerun in comparison (it too, was the second "monster book" of its edition, but with only 96 pages!), and a substantial amount of the creatures in there are actually older edition monsters that simply didn't make the 3e MM and were updated here, even though they weren't particularly FR-specific. In actuality, a good number are even in the 5e MM!
I honestly don't think we'll get much on dragons in the book. But then again, since they are half of the name of the game, they will need to make at least a token appearance. I'm thinking we'll see one or two dragon types updated (most likely the deep dragon at least), and maybe a few examples of premier Realms dragons that can be easily adapted to other settings simply by re-naming (heck, they could re-use three very nasty spell-using evil dragons from SKT if they wanted). I'd love to see the gem dragons updated, and still hold out some hope, but it will take around 15 pages or so for the all stat blocks and quick descriptions, so I'm not sure if they will use that much of precious space in this book to do so, especially if they have a psionics book in the pipeline.
Giants, on the other hand, I think we'll see a bit more of. We already know we're getting firbolgs in the book, and one of the developers stated we would see other giant variants as well. Again, SKT could be the inspiration for such variants as cloud giant mages and frost giant jarls. One or two more powerful variants for each giant type probably wouldn't be unwelcome, but I wouldn't want to see much beyond that.
I'm kind of leaning towards the opinion that we won't be seeing too much in the way of extraplanar creatures in the book, so your fear about demons and devils will hopefully be unfounded. Anyway, when it comes to devils, there aren't too many more types to update (the abishai are the most notable of the missing, and as they were in
Monsters of Faerun I wouldn't be surprised to see them in VGtM) I have a feeling we'll see a few types of the various fiends (and whether the demodands/gehreleths will be updated, who knows?), and maybe a bit more focus on the celestials (given their near-absence in the MM) when it comes to extraplanar beings.
).
