Considering Lord Harkon was a bit older than 4000 years old from the SKYRIM: Dawngaurd DLC lore I imagine he is around 3000 years old in ESOs time, which means he must be alive and well in Skyrim. I hope we can do quests for him if you are a vampire, or something in that Idea range and find castle volkihar if it exists. If this is not already in the game I am hoping it will be in an expansion or future patch. Anyone with Ideas or theories is more than welcome to comment please.