So there's finally a Dev Diary up for The Old Gods.
http://forum.paradoxplaza.com/forum/showthread.php?676756-Crusader-Kings-II-The-Old-Gods-Dev-Diary-1-A-Different-Europe Not much is said but there's some interesting things already. There are now some navigable rivers which seem to be thin sea-zones running through landmasses which some mods have done already but still quite cool. There's now portages which work like reversed straights so I'm guessing we'll finally see a path to the Red Sea.
Pagans seem to have a different polygamy to Muslims as they can have a single wife and several "concubines" who's offspring may or may not inherit claims/titles, it's not clear on that yet.
Judging by the screenshots it appears that the Norse now have the ability to loot settlements and then drag that loot back to their lands, loot actually takes up space in galleys! Not only is this interesting by itself it might indicate with some modding magic we'll be able to have interesting pirates/sea bandits sea bandits terrorising the coasts!
There does seem to be quite a few problems with the map already though. I understand it's not meant to be a full Dark Age simulator but rather CKII dragged back a few years but it just seems plain wrong to have Scotland, HRE and Burgundy in 867. Iceland is apparently also ahistorically colonised so we won't see any new events or mechanics that modders could cannibalise sadly.
I'm hopping that the Celts aren't generic Catholics (judging by their shields they seem to be however). They should really have their own Celtic Christianity heresy for this time period which should include polygamy at the very least.
I'm also hoping they Pagans can reform their religion as well. Not only converting but changing it to a more defencive insular format. It would be extremely silly to load up the 1066 start and see the last few Norse pagans still raiding settlements.
I'm still a little skeptical about the DLC. I still think there was a lot more that should've been added to the 1066 start before they rolled back a couple centuries. I also think that now they're forced to balance for two different time periods and make sure mechanics make sense for both. Still it could be a bit of fun but it's likely going to disappoint a lot of fans of the time period who are wanting a full late Dark Age simulator.