As much as I prefer Morrowind's mystical and hopeful tune to Oblivion's frantic, blaring cacophony of trumpets, I certainly wouldn't ask for a direct rehash of either one. Again, I feel that many of us need consider that, sad as it may be, the province of Morrowind is certainly unparalleled in its exotic sights and sounds. I'm hoping future TES games will be nowhere near as generic in theme as Oblivion, but I doubt we'll be seeing many more giant insects or mushroom towers of any sort.
So what do I want in a main theme? Something meditative and envoking, with good progression (most Oblivion songs have 10 seconds of memorabilia and then it's the same sad soft strings crap for 3 minutes), something that makes you feel hopeful and thoughtful. But perhaps it could be a bit more uplifting and a bit less reflective of Morrowind's "hopeful, triumphant return of Nerevar" feeling. Just because no two Elder Scrolls games are alike.
EDIT: Of course, http://www.youtube.com/watch?v=I_MY0ihuJZc has shown us that generic classical music is a safe fallback as well, though I would hope Bethesda is above that now.