If indeed this is the new campaign setting Wizards has been teasing for almost a year, there are a few places it COULD be. I would agree with the majority of you that Planescape seems to make the most sense.
Ravenloft is also possible. Curse of Strahd seemed to be popular. And while I'm not sure if WOTC would want to update the rest of the domains for 5e since their most famous part of Ravenloft IS Strahd, I think a setting book would do well. It would also be nice to have a book with updated rules for checks like fear, horror, power, etc.
Honestly I would be thrilled with either Planescape or Ravenloft.
Ravenloft is also possible. Curse of Strahd seemed to be popular. And while I'm not sure if WOTC would want to update the rest of the domains for 5e since their most famous part of Ravenloft IS Strahd, I think a setting book would do well. It would also be nice to have a book with updated rules for checks like fear, horror, power, etc.
Honestly I would be thrilled with either Planescape or Ravenloft.