There should be no more classic after WotLK.
the point of classic was to experience the content of the game that couldn't be experienced anymore.
Wrathgate is more than enough reason to have a classic WotLK, as its a legendary event and quest chain. To get there, we had to do TBC.
Vanilla - TBC - WotLK also tell a final complete story, ending the WC3 chapter.
Cataclysm was when WoW pretty much became WoW 2.0 --- it doesn't deserve the classic treatment, as the entirety of its game exists in retail.