Originally Posted by
ablib
It depends on how Blizzard handles it. If it's legit just like a private server, then you don't need to worry. It will be popular at first, but it will quickly die. If it requires a sub, it will also quickly die.
However, I'm sure Blizzard will create some incentive or purpose to play on a Vanilla server, to make the experience worthwhile. If this is the case, then yeah, it could pull from the latest game.
It would be a stroke of genius on their part if they released this at the same time as the last raid of Legion, to give people something to play with until the next expansion comes out.