I've been wondering this more and more..
Silvermoon and Quelthalas being updated to an expansion size zone makes sense in this regard, but Northrend is already massive. I don't see them cutting zones out so to speak, but I don't know how they are going to bring relevant storylines into the fold with all the area space we're aware of.
I don't see them doing an underground expansion again, but I do think its possible we could be going underneath Northrend instead. Azj'ol Nerub and lots of Manifold zones could be possible. There is a very small part of me that thinks the expansion intro alone will be in Northrend, and we lift a section of Ulduar into the air akin to Odin as a home city after the intro cinematic, in order to go somewhere else to find a possible "Last Titan" elsewhere in the world. However I do think that is very very unlikely. I doubt they'd drop returning to Northrend and then not truly return.