The DLC would be like the Shivering Isles only that you could actually travel different planes of Oblivion. The way you could travel to Oblivion is using a new master Conjuration spell Spell Tome: Open Oblivion Gate. Disregarding that Martin Septim sealed the Oblivion Gates and sealed off the Daedra. It did not say however, that a mortal could open a gate to Oblivion via spell. Once you enter the gate a text box of options will open and will show options of the planes of Oblivion. Ex. Moongleam, Coldharbour, and Deadlands. The DLC will be like a gauntlet style match. Once you defeat the lesser Daedra you face the Daedric Prince of that plane and once you defeat him or her. You gain enough Magicka to open another gate. You would start from Peryite's plane all the way up to Mehrunes Dagon's plane of the Deadlands. Once you defeat him, you will have the options

Become Daedric Prince.

Become Divine.

If you choose Divine the DLC ends and you have over-maxed skill rating by a factor of ten. But, if you choose become Daedric Prince. You can literally kick out a Daedric Prince and become him or her and rule that plane. Then you could invade Tamriel once more by teaching enough Daedra with the spell. After you start invade Tamriel with your army of Daedra, the Divines become outraged and Azura and Merdia will abandon your alliance to fight for the Divines if you had not kicked them out. You could follow them and reverse your invasion. Thus making a huge battle near Whiterun to push back the combine armries of the other Daedric Princes. If you do succeed in that path you keep your Daedric powers and everything becomes well again. But, if do not side with you could crush Tamriel's proviences one by one.

So what do you think about it, huge enough?