I don't know, it has in theory but i simply feel like there's the massive issue of rewards looming over this.
Classic, in itself, is a finished game, while there is certainly stuff in there that could be improved and such, overall the game isn't exactly incomplete.
The big problem here is, new stuff needs new rewards, new rewards will either replace existing stuff or be considered useless.
Say you add a new 5man dungeon, if the gear that drops in there is too good, people will just spam that dungeon, if the rewards aren't good enough, people will quickly ignore it (for example, it became more difficult to do regular max level 5man dungeon post ZG / AQ launch, as a lot of people simply went into ZG / AQ with their fresh alts).
Similiar thing goes for raids, let's say there is another Raid *after* Naxx, then what?
Another after that?
Should the "new raids" be wedged inbetween?
That means the new stuff is being made obsolete by the older "content".
In my view, any direction of Classic+ should be rather focused on polishing Classic because i think there can be a lot of stuff improved that doesn't necessarily can cause issues of the magnitude that new content can.