How would a Forsaken expansion work? One feasability that I see would be if they completely revamped the Northern Eastern Kingdoms to become the new 'continent' of the next expansion, revolving around some evil force beneath Tirisfal Glades, idk.
Some new zones:
Quel'thalas (Eversong Woods, and the Ghostlands reclaimed by the Blood Elves and looking less dead, Silvermoon City finally complete)
New Lordaeron (East and West Plaguelands together, secured by the Silver Hand, but Kel'thuzad is back and the Cult of the Damned has a vice grip on Stratholme)
Tirisfal Glades (Tirisfal Glades and Silverpine Forest together, here be Forsaken plot)
Gilnean Peninsula (It's Gilneas)
Something with Hillsbrad and Hinterlands combined
Something with Arathi Highlands. I gave up with this suggestion.
But yeah basically transforming everything north of the Thandol Span into the staging ground for a new expansion, yeah sure it would definitely work. Though it would have to be phased so that it's not accessible to sub level 100's.