Hey, I was just thinking of the possibility of Harkon and the Volkihar clan being a part of the Elder Scrolls Online story.

As one of the main antagonists of the game will be Molag Bal and the vampires featured in Dawnguard are main servants of Molag Bal, there could be a connection.

Also, remember that Serana does not recognize the Empire when the player mentions it when you rescue her. In the Second Era the Empire is not in power as there are many factions fighting for the throne. This could easily be a reference to an earlier time period, but it could fit.

The location of the Volkihar Castle could either be in the western, Daggerfall Covenant half of Skyrim or it could be within expansion territory.

It's pretty new to have different parts of the Elder Scrolls Universe being created by two different studios at the same time, but do you think
Bethesda could be setting up hints and nods to the MMO?

I wouldn't be surprised if they are included in the MMO. The Dawnguard (the original faction) will probably be featured too. The http://www.uesp.net/wiki/Skyrim:Dawnguard_%28faction%29 page says the fortress was originally built in the Second Era to contain the Jarl of Riften's son after he became a vampire. There will probably be an Ebonheart quest involving this.