I dunno. If they continue the path that they're on now, I'd suspect that the next storyline release would be back to the FR, since they wouldn't be sure in advance how Curse of Strahd was going to go over with the audience. We've had Dragons and Elemental Evil and Demons, and CoS is kind of an Undead adventure, so I'd suspect some kind of adventure against Giants or Aberrations in FR would be next because they seem to be covering a lot of the classic foes.
I think before we get an Eberron sourcebook, there will be an Eberron adventure. And before there's an Eberron adventure we'll get one or more plane-hopping adventures. The current structure seems to be to reinforce AL play in game stores and at home, and so having non-FR adventures where the players can be off-worlders who come to that setting (like Curse of Strahd) is easier to fit into that structure than releasing adventures for an entire new world that would need to be supported.
OTOH - I wouldn't be surprised to see another Eberron web supplement/Unearthed Arcana at some point.