Fingers crossed for Mannimarco (the REAL Mannimarco mind you, I have a mental retcon in place) returning in full force in a future TES installment. A lot of potential there...reemergence of Necromancy, player liches, that sort of thing. I know It's unlikely because of oblivion and he will likely be in the limelight for TES Online, but all powerful lich lords seizing power could make for an awesome atmosphere.
On that note, I want to point out something. In TES there is always some sort of unseen "rising threat", but I think it would be cool to have a game that takes place in an era of subjugation (Skyrim gave us a taste of this with the Aldmeri Dominion, but they weren't the primary antagonists and their presence was more inferred than experienced), I don't want Mannimarco to just be an illusory threat dangled over us, I want him to to be a political force, with dark wizards serving as tyrannical rulers over a terrorized populace that live in terror of being used in a compulsory necromantic experiment, or slaving away excavating ancient burial sites and ruins. Liberating this corrupted empire would me be a much more perceptible "uphill struggle" and give a sense of ancomplishment with greater indication of your heroic feats. Your not going from intact, boring empire to "YAY our empire still isn't destroyed!", but from Magical oppression to sunshine and rainbows. And unicorns. Or something. Now that I think about it, I want something like this regardless of the who the big bad is... Borderlands 2 taught me that a villain is much more memorable when they are an "in your face" kind of jerk.

Well, that's my idea of a good time, what kind of big bad/tragic situation what you like to see play a major role in TES?