The third era is ending and everyone knows it. The governments of The Empire are falling apart, and have been doing so ever since the Nerevarine destroyed the Heart of Lorkhan. The tribunal's power is gone, the Nerevarine has vanished, there is no Emporer, and the only thing keeping the Empire and all of its governments together is the people's belief in that government. This has been demonstrated in Morrowind through the disbelief among the citizens when told of Almalexia and Sotha Sil's demise. In Oblivion, the people are worried and that belief in government is beginning to crumble, and several forces are looking at Tamriel like a lolipop in the hand of a baby (Mainly the daedra and potentially Akivir).
There have already been (possibly) two attacks on Tamriel by the Daedra. There's obviously Mehrunes Dagon, but I believe that Hircine may have started to make a move in the Bloodmoon storyline. However, I feel that it was him having fun, but if he saw an opportunity, he would have jumped on it. One other daedra who I think has an eye on the land of Lorkhan is Azura. After all, what did she have to gain in the destruction of Dagoth Ur? What does she owe the Dunmer people? Aedra or not, I got a feeling in my most recent playthrough of Morrowind that she's very subtly stacking her ducks. She, in a sense, debased an entire religion, obliterated any opposition through the sixth house uniting Tamriel under an iron, ashy fist, and her followers have increased in number. Whenever she did appear to you, it was kind of like staring at Almalexia for the first time, thinking that there might be something else to her. After all, she does work in mysterious ways...
I think Tamriel's future lies with the Daedra and their followers. The eavesdropped rumors in Oblivion point at it all over the place. Daedra worship increasing in the Summerset Isles and House Dres is starting to gain power in Morrowind. That's where I think we're heading, however, the story could go in many directions, Akivir could invade, the dwemer could return(that would be a hoot), a third numidium comes out of nowhere and causes some ruckus. But I'm not Todd Howard. So I don't know....