I feel like this is the one thing missing from D4.
However, wouldn't something like a secret level with unique monsters show up in the data mining? Unless blizz decided to go out of the way and make that specific feature completely server-streamed.
Admittedly I haven't watched the video yet as I'm at work.
When I found the fountain it made me wonder if their was going to be a cow level.
It also makes me wonder if there is going to be a way to return to The Vault from Diablo 3. There is a quest that makes mention of the vault, so I am hoping it makes a return with the cow level.