So the pseudo-ARG thing they have going is starting to wrap up. It seems almost undeniable that the picture is of a Norse soldier standing in front of a longboat. This seems to indicate that it's going to be a Norse DLC (unless there's some bastardly misdirection). There is still a chance that it will have more than just Norse pagan and/or Northern Crusade content.
I'm personally starting to believe that it will be an extremely small DLC focusing just on the Norse faith with the season-changing-map-mechanic that EUIV is boasting. Unless it's been in development before The Republic I really just don't think we'll get anything bigger than a flavour DLC.
The season changing would of course be a lovely addition to the map and should (in theory) be easy enough to port over since the two games apparently run almost exactly the same engine version. However on the other hand it could be seen as a stupid move to add a mechanic that is so heavily marketed in their next game.
Possible other (and not likely) possibilities/mechanics include:
New fantasy DLC. Whoo.
An earlier starting date to when Vikings were a thing. However history does start to get rather iffy before 1000AD.
Northern Pagans/Northern Crusades.
Vinland/Greenland map expansion. Gives Iceland an excuse to become a merchant republic since they supported the colonies so much. Rather unlikely since you'd technically need to add colonial mechanics.
Navigable rivers. Can't have Viking raids or Viking trade without navigable rivers.
Naval Comb- HERESY! WAR GALLEYS NEVER EXISTED, ASK PARADOX!
Improved coastal assaults?