To me, Giants can be a good book like Fiztban is, so I'm interested in it.
Planescape can be a good book to expand the DMG information about planes, until it is something like planescape giving only an adventure, a city, some generic rules and a monster book.
I'm interested in the book of many things: I've the feeling that will be a book about managing a medium/high tier campaign. Like domain management, castles and war. Still light as in 5e fashion (no War Machine or Siege Engine), but still something official.
And naturally, none is in my "to buy" first. First WotC clears it's name, then I will buy it. Simple as that.