During BlizzCon, Blizzard showed off the expansion The Burning Crusade for World of Warcraft Classic. Just like 14 years ago, you go to the Outlands to go on new missions and fight new monsters.
Some small changes will be offered from the original though. Among other things, the raid bosses of the expansion have been made more difficult to defeat because Blizzard assumes that everyone has already killed them countless times.
When The Burning Crusade expansion will be released for World of Warcraft Classic is unclear, but sometime this year it will at least appear. It costs nothing extra if you pay monthly for it.
A proper deep dive into the expansion can be seen below.