Ahh, that's interesting, I either didn't notice that, or had forgotten. (I tend to skim the adventures rather than read them cover to cover)
The monster books like Volo's have also covered some race options in the past (less so in Mordenkainen's but still there)
Would there be enough player options for a horror themed book? I guess they could give a bit more detail on playing lycanthropes?
I ran Dragon Heist awhile back, so I remembered that plot device quite well. In the campaign, the players helped him write/research aspects of it, helped finance it, and even got to put notes into the book. It would be nice to have a physical version of it.
Ehh, it depends. I don't know if we need many more races, as we have most of the main/drawing races down (IMO). I expect most races to come in setting books. For example: Thri-Kreen and Mul could come out in Dark Sun, Kender and Wild Elves (maybe even Draconians) in Dragonlance, Kor and Vampires in Zendikar, and so on.
If a Draconomicon is coming out, I don't expect any race options to come out.
Maybe (and this is very unlikely, IMO) there could be a Half-Dragon template balanced for PCs, but I highly doubt this will come (I think a Dragon-Touched feat would be more likely). If a Draconomicon has player options at all, it's going to be the Ascendant Dragon Monk and Drakewarden Ranger subclasses, and maybe a few other subclasses if they decide to make them (Dragon Barbarian, Warlock, and Fighter would be cool, maybe even Druid, Paladin, or Cleric). Edit: There could be "Gem Dragonborn" options, but that's probably not coming.
If it's a horror-themed monster book, they could make a Vampire/Dhampir race, put Shifters as the "Lycanthrope" race, and maybe add an aberrant option (I could see them having Kalashtar be the aberrant option). They would then add in the Undead Warlock and Spirits Bard subclasses, and possibly the Spores Druid, Beast Barbarian, and Aberrant Mind Sorcerer subclasses.
I personally doubt that there will be a "Horror" themed monster book. IMO, it's probably going to be a Ravenloft/Innistrad setting or adventure compilation book and a Draconomicon (X's Guide/Manual to Dragons or something like that).