They've managed, intentionally or unintentionally, to pull some damn good fridge brilliance before the game has even come out. Out of all the guilds announced, there is information on both the Fighter's Guild, and the Mage's Guild. But the Dark Brotherhood and the Thieves Guild strangely have no information available at this time. Let me repeat that, the Thieves Guild and the Dark Brotherhood have no information at this time. The two most secret guilds in the Elder Scrolls universe.
The two most secret guilds that we have been able to join. Beyond organizations so secret we aren't aware of them, the Dark Brotherhood honestly isn't that much more secret than the Necromancers.
In the Maormer Ambassador, Ulondil's quarters in Mistral there's a copy of the book Wardens of the Green. While it seems innocent at first, you later find out that he was planning to murder them both and assumed because of how the text was written that killing one of them would kill them both at the same time. He panics when he finds out that wasn't the case.
The Ebonheart Pact is the only one of the three alliances that does not give you a quest line to convince a neutral territory to join your alliance. This is because the Pact is the longest standing of the three alliances, and has had 11 years to recruit territories, unlike the newer Daggerfall Covenant and Aldmeri Dominion.
There's a small example in the symbol on the game's box art, regarding the placement of the animals each faction's insignia. The eagle of the Aldmderi Dominion is loated at the bottom. The Dominion consists of the Summerset Isles, Valenwood, and Elseweyr, the southernmost regions of Tamriel. Similarly, the Daggerfall Covenant's lion appears on the upper-left of the insignia. As anyone who has played Daggerfall knows, the Covenant extends across Hammerfell, Orsinium, and High Rock, which extend up the west coat of the continent to the north. And lastly, the Ebonheart pact, whose dragon is seen on the top-right, covers Skyrim, Morrowind, and Argonia/Black Marsh, which cover Tamriel's Northwest borders. Additionally, in regards to the symbol's O-shape (Which obviously also means Online), all three factions are fighting for control of Cyrodil, which is currently ruled by the Alessian Order, and worships a god associated with Oblivion.
One thing that might be confusing to players is how at the end of the main quest you get your soul back, and yet when you die you can still revive like normal, even though you aren't soul shriven anymore. Players of Oblivion with the Knights of the Nine DLC might remember that Umaril the Unfeathered made a deal with Meridia and could reform and revive himself an unlimited number of times, yet could only be killed permanently while he was reforming. Meridia is basically doing the same thing here with you.
In the course of five games, Molag Bal, the Daedric Prince of Domination, has shown himself to be quite possibly the most evil thing in the franchise. And do you want to know something else? He's only ever appeared in sidequests! That right, folks, he's been a distraction before, but with Online, the King of Rape now has his undivided malevolence set upon you. If you thought the atrocities he has committed as a footnote were bad, just how bad do you think he'll be when you have his full attention?
On the other hand, as Fridge Brilliance, this justifies the MMO itself. What do you do when you're facing the local embodiment of evil? You send in The Chosen Many.