You know in morrowind how they said that that the dark brotherhood had significant presence in Mournhold? (and thus the entire Tribunal expansion pack)
Well, what about that? you could dig up all the old Mournhold maps, re-build it for Oblivion, and you could have a Morag Tong/Dark Brotherhood war. The problem with this would obviously be that the Morag Tong is a law-abiding organisation that hunts down criminals (or something like that i don't remember), but you could have it so that the Morrowind government wants you, the listener, killed. a Morag Tong assisin shows up whenever you wait or sleep in a 10 gold or less inn. when you kill an assasin he's carrying his writ of execution with him and when you read it, you get a message hinting that you should report this to the speaker in the cheydenhall sanctuary. she says that this is serious, seeing as how your identity is a closely gaurded secret and there must be a leak in the Brotherhood. she then directs you to meet her and the other speakers at the statue of the Night Mother. They open the tomb and the night mother (after some long and greuling speach) decides to send you to Mourhold, where you meet up with the local Dark Brotherhood members, who have been just recently informed or theidentity of the new Listener (you). There have been increased tension between the Dark Brotherhood and the Morag Tong recently, and a Dark Brother was captured during an assasination attempt of the Morag Tong leader, and he must have ratted you out. It was a one person job because the only opening in thier defences was only allowed for one person. They have, since the assasination, fixed the only hole that the Brotherhood was able to find. you end up having to use the local Theives Guild for help, and they'll only help you if you are the Grey Fox, and wearing your guise. the Brotherhood ends up doing quests against certain people in Mournhold, trying to find information that could lead to the discovery of another gap in thier defences. these are non-lethal force missions, using a teqnique called "interrogation." You hold them at knife point, and extract information using special potoins that make them more agreeable to talk. Once they talk thier memories are altered so that they have no recollection of the encounter. the Guild breaks into certain places and steals blueprints and other valuable documents that were deemed "helpful" by the people you interrogate. they are copied at the local Dark Brotherhood sanctuary and have to be put back before anyone notices, or it will raise suspission. the end result is the realization that there is only one way into the Morag Tong HQ, and that is through the front door. However, this is obviously well guarded, and nobody is allowed near it since the assasination attempt, so you have to do something drastic. the entire Black Hand and most of the Brotherhood show up and in one incredibly eventful night, the Dark Brotherhood kills every member of the Morag Tong in Mournhold, thus crippling the Tong throughout Morrowind, aiding in the invasion of the Brotherhood into Morrowind.
It would be an insne amout of work

, but throw in a few fights in the streets, and you've got the ultimate Brotherhood mod.