UPDATE: Feb. 19

The expansion was officially confirmed during Blizzcon 2021 opening ceremony.

WoW Classic: Burning Crusade will likely be one of the major announcements at the upcoming BlizzCon 2021, according to an info sheet that was published, and later removed, on Blizzard's official website. According to the leak, players will be able to choose their eras, a.k.a choosing whether they advance to Burning Crusade and the world of Outland, or stay in Azeroth and play the original WoW Classic. 

The leaked info sheet, screenshot by Redditor Urangutansupremacist, reads:

"Journey through the Dark Portal once again in World of Warcraft®: Burning Crusade Classic™! Originally released in January 2007, The Burning Crusade summoned the heroes of Azeroth to the shattered and fel-scarred realm of Outland to stop an invasion of the demonic Burning Legion. In 2021, Blizzard Entertainment’s recreation of the first World of Warcraft expansion will give players from around the world a chance to return to Outland as it once was to relive an era of timeless adventure—or experience what awaits beyond the Dark Portal for the first time.

• Rediscover the Broken World of Outland – Flee the fel reavers who roam Hellfire Peninsula, dive deep beneath the swamps of Zangarmarsh to confront what lurks below, and clash with the demonic agents of the Burning Legion in the shadow of the Black Temple.
• Join the Fight as a Blood Elf or Draenei – The ranks of the Horde and the Alliance grow! Fight for the Horde as the blood elves, seeking a new source of the arcane power that once sustained them, or join the Alliance as the draenei, exiles from Outland in search of a new home.
• The Saga Unfolds Over Time – Content from the original game will roll out in phases, at a cadence paced for the WoW Classic community. Prepare for the opening of the Black Temple, gear up to confront the gods of Zul’Aman, and gather your allies to face the fury of the Sunwell.
• Burning Crusade Features Reborn – Prove your prowess in the Arena PvP system, enhance your gear with the Jewelcrafting profession, take to the skies over Outland on flying mounts, choose whether to seek the aid of the Aldor or the Scryers of Shattrath, and much more.
• Choose Your Era – Prior to Burning Crusade Classic’s release, WoW Classic players can decide whether to advance each of their characters to the Burning Crusade era with the rest of their realm, or to continue playing the original WoW Classic content on new Classic Era servers.
• Included With Existing World of Warcraft Subscriptions – As with WoW Classic, anyone who subscribes to World of Warcraft can also play Burning Crusade Classic at no additional cost, giving players the freedom to enjoy multiple eras of Azeroth at their own pace."

All images by: Blizzard