The reason that Classic+ is an interesting idea is that Classic Vanilla is a completely different game from the expansions. It isn't hyper-focused on a handful of zones. It's a large world. They very well could have gone in a different direction (horizontal world expansion) rather than the "new expansion, new continent, goodbye old content" model they went with, and devs have confirmed that this was a consideration initially.
Once you are in any expansion, even TBC, this makes no sense anymore because the model has already been decided.