All the players who left World of Warcraft will come back for World of Warcraft: Classic, which will show up in the subscription numbers in the earnings announcements.
This will also get a boost from Blizzard's mobile game Diablo Immortal, which will be most likely be released in 2019 +/- few months between the release of World of Warcraft: Classic. It will be also developed with a Chinese company NetEase, so it will get released in China. Keep an eye on NTES NetEase aswell, around that time.