My idea would be The Oblivion Planes. Simply, it is a large area of woodland in Skyrim's remote regions (probably along the Northern coast) that has been felled and cleared. When you go there, you discover about twelve different characters, each one representing their deity's plane. They are sat around a large table with a few (unreachable) Daedric artefacts. When you talk to each priest, they will tell you about their Daedra that they worship and then ask -you to go on a quest for each one to activate their portal to a small portion of their plane, for worship.
The Mehrunes Dagon priest will show you to a closed Oblivion Gate, and asks you to acquire a full Black Soul Gem, three Daedra Hearts and an Ebony blade of any kind to open the gate. - The inside would be the same as Oblivion, perhaps with one or two new Daedra. Instead of a Sigil Stone in the Sigil Keep, there will be a high-ranking Dremora on a throne for the player to defeat.
The Sheogorath priest asks you for twenty sweetrolls, a Grand Soul Gem and a Daedra Heart to open up the portal, which would probably be the same as the Shivering Isles portal. (The three faces with a portal in one of the mouths) - The inside would probably be Passwall from the Shivering Isles add-on, with a couple of caves and forts with typical Shivering Isles enemies to hide, such as Grummites, Hunger and Skinned Hounds.
The Hircine priest asks for the remains of the lands mightiest creatures. You are asked for a Bear Pelt, Cave Bear Pelt, Snow Bear Pelt, Sabre Cat Pelt, Snowy Sabre Cat Pelt, Dragon Bones and Dragon Scales. Also a Grand Soul Gem and two Daedra Hearts. - The inside would be large foggy fields with an abundance of life to hunt, such as Bears, Sabres, Deer, maybe a dragon or two and probably (for higher levels only) a werewolf to fight, too.
Those three are just examples, there are sixteen Daedra Princes after all, not that they would all be in this add-on, maybe just five to ten or so.
So what do you all think of my idea?

