Namely, World of Warcraft Classic is getting the Token system which the players
can use to purchase monthly subscription to the game. The reason why
the fans might be reluctant to have it in Classic is that the Tokens can
be purchased for real life currency and sold on Auction House for
in-game monetary gain.
Combined with the fact that those who enjoy WoW Classic are in it for the feeling of enjoying the game as it
used to be back in the glory days of vanilla, The Burning Crusade and
Wrath of the Lich King, having the Tokens appear and ruin in-game
economy would be disastrous.
However, the Tokens have been announced only for China and since the region has its own client, it
will not directly interfere with the experience of the western players.
At the time of writing, Blizzard did not confirm nor deny the potential
possibility of adding Token to WoW Classic in other regions and
considering that the players are highly unlikely to enjoy its addition
in the west, there is still a chance the company will not go down this
road.