Latest since the leak of Blizzard itself is clearAfter Vanilla WoW, The Burning Crusade will now also get a new edition. The official announcement of TBC Classic was only made now, in the course of the opening ceremony of BlizzCon Online 2021. Below we have summarized all the details known so far.

• The current WoW Classic servers will be expanded to include content from The Burning Crusade. At the same time, Blizzard plans to offer new “era realms” to which WoW Classic players can transfer their characters, which will continue to roam only in the WoW Classic ecosystem.
• Those who want to dance at both parties can do so as well. There will be a paid copy service that will allow you to copy your Classic character from the TBC Classic server and then transfer the clone to an era realm.
• Players who haven’t played their character to level 60 in WoW Classic yet, but want to rock Outland, can take advantage of an optional level 58 character boost. This will be available before the launch of Burning Crusade Classic. This boost will not be available on the era realms or for the new races (Blood Elves and Draenei). Only one character per account will be allowed to reach level 58. More details on pricing and availability will follow at a later date.
• TBC Classic (like WoW Classic) will have several phases and no progressive patch cycle (as some players had wished for).
• The Classic servers, whether TBC Classic or “Era Realms,” will continue to be available to all World of Warcraft subscribers at no additional cost.
• We still have to wait for a concrete release date. TBC Classic is scheduled for release in 2021. The beta is to start “soon”.

Here’s the info from the first WoW “What’s next?” panel, which was solely about the implementation of TBC Classic (umpteen surprises included):

And here you can see the announcement trailer for TBC Classic and some official screenshots from Blizzard: