There are no limits to the current engine, they are always extending the game engine every expansion pack.
Launch wow didn't have:
Current view distance, which has been extended at least twice.
Current water details, the reflections and the waves were added to the engine in Catacylsm.
Current dynamic shadows, the shadows from characters and from terrian was added to the engine in Wrath.
Current streaming downloading, this was added in MoP when a new launcher was made so you could log in with the bear minimum of game downloaded.
Current game files, the game launched using MPQ files from warcraft 3 to store all the game data, it was changed to the new CASC system in Warlords:
https://worldofwarcraft.com/en-us/news/13645405
Every expansion expands the game engine if there is something the developers want to add or expand on.
The only limit is what the devs want to do, I don't think they've ever mentioned limits or "WoW 2". Except from people expecting a "WoW 2".
Lets be honest. The Burning Crusade was WoW 2.0 like Battle for Azeroth will be WoW 8.0.