If you ever played Final Fantasy VII, then you'll remember Ruby Weapon (or to a lesser extent, emerald weapon).
I think Emerald was harder, but it just depended if you could find the trick on how to beat them.
Both tricks were very strange since it involved giving your own team a disadvantage.
Back on topic: I'm also for deviously designed characters carrying a powerful artifact. Something like seven legendary warriors, all at least level 50 with epic gear...
I think it would also be nice if some really difficult quests in general opened up only after you beaten the main quest or a faction.
One week later after you finished the main quest someone approaches you looking for "the most powerful warrior in Skyrim" for some reason.
Or when you're the Arch Mage one of your Master Wizard seeks your advice on something he's working on, leading to a "master quest" with an exceedingly difficult puzzle where you need to find clues in several dungeons, books, etc...
Just give us more endgame material...