The Old Gods expansion also provides an 867 AD start date, right at the height of the Viking Age even though it's been largely superseded by Charlemagne's 769 start, it's still two centuries more of history per playthrough.
With this expansion, large swaths of the map become playable, and where there's large swaths of map, there's a need to get the DLC that unlocks them. Want to fight back the advance of Islam and restore Zoroastrianism in Persia? You guessed it, Old Gods. Want to play as a Lithuanian or a Finn with the Romuva or Suomenuosko faiths? Old Gods. Want to play as the Mongols? If you want Tengri factions, the religion of the early inhabitants of that part of the world, you'll need The Old Gods. This isn't just author bias: The Old Gods is the gateway DLC that makes just about all of the other major CK2 DLC packs that aren't about the monotheistic religions work.
With that in mind, we'll go ahead and rank all 15 DLC releases from “if you don't have this DLC, you're missing out on the true value of the whole game” to “don't bother”. And while the answer to this is “depends on your playstyle”, there are nonetheless certain DLC packs that add so many great features to the game that it doesn't matter if you're playing as a Norse pagan in Sweden or a Brahmin in India, you're still going to love what the DLC has to offer.
