WoW TBC Classic: Blizzard is already identifying what they need
During an AMA on reddit with the WoW Classic dev team a user asked about potential plans for after phase 6.
What are the plans after Phase 6? More content for classic or moving to TBC?
World of Warcraft Executive Producer John Hight took the answer there:
We've done most of the hard work by bringing back 1.12 so progressing to Burning Crusade would be a lot easier for us. Our plan is to identify everything we need to do should we ever decide to go this route. We want to be sensitive to the desires of our players. Some may want BC and some may not. We'll be following the Classic community closely to help determine what our next steps should be.
Based upon his reply it sounds they are already tentatively planning for Classic version of The Burning Crusades expansion.