World of Warcraft: Classic
The goal of World of Warcraft: Classic is to let players experience World of Warcraft as close as possible to as when it was released. Before work began on World of Warcraft: Classic, it was only possible for players to experience the original World of Warcraft by using emulated servers, most of which had stability or corruption problems.
As much as Blizzard has been aware of the desires of their community, until recently it seemed impossible for them to emulate classic servers due to the technical hurdles of essentially having to run two massively multiplayer online games side-by-side. A breakthrough was then achieved that made it possible to run classic servers on the modern architecture of current World of Warcraft servers.
Even though a modern server architecture is used, classic servers won't have the same features that current World of Warcraft does. There won't be cross-realm servers or Looking For Raid and Dungeon Finder automatic party matchmaking. There's still a lot of questions about how the team will tackle it. This endeavor is being undertaken by an entirely separate team at Blizzard from the one working on World of Warcraft and its next expansion.
As of June 2018, the team settled on using patch 1.12 as a foundation for World of Warcraft: Classic. They managed to have a locally rebuilt version of Patch 1.12 running internally, using modern code and data architecture.