At Blizzcon 2018's opening ceremony this morning, World of Warcraft executive producer John Hight announced that the upcoming recreation of World of Warcraft's original release, World of Warcraft Classic, will be free to anyone with an existing World of Warcraft subscription.
World of Warcraft Classic is currently planned for release in summer of 2019 and a playable demo will be available at Blizzcon, or to anyone with a Blizzcon Virtual Ticket.
Classic was announced one year ago as an answer to the growing problem of fan-hosted legacy servers for the game. At the time, then-executive producer (now-Blizzard president) J Allen Brack said that Classi would "replicate the game experience" of Vanilla World of Warcraft, but would be adapted to work better on modern technology.
Some more modern features that have been confirmed already to make it into Classic include colorblind mode, player reporting, Battle.net chat implementation, and widescreen monitor support.