Imagine clearing a room of skeletons, thinking to yourself that you are now safe because you cleared the whole dungeon. You walk over to look a chest and as you exit loot a scream comes from behind you, a lich rushing you trying to kill you.
Picture yourself working your way down to a boss spider in a cave. Most spiders up until now have been no bigger then a large dog, but now you face one the size of two men. You engage him in a fight and slay him, then feeling good about yourself you shake off the blood of the battle, turning around you stand in horror as you see a spider descending down from the ceiling, twice the size of a elephant.... Do I run? If I don't do it now he will stand between me and the door.... Or can I defeat him?
Stuff like this would be cool, it would add a lot of flavor. You couldn't do it all the time though, it would have to be rare enough that when it happened it felt like a epic battle, but it also had to be hard enough that you would have to question whether or not you should run or fight. Make me scared to face opponents, until I gain power of course. You talked about how advanced your engine is, maybe you could make some kind of system that could randomly generate epic dungeons, and as you cleared out the other dungeons they would have a chance of spawning a epic dungeon.
I am sorry but I found the dungeons in oblivion to be boring at times. The bosses were already spawned and you saw them before you fought them. The only dungeon I loved in oblivion was the one across from the jail where you exited and you fought bandits, oh spoilers ahead btw, and at the end you fought the boss. For years I never knew that there is a hidden lever in there and if found it unlocks a door to a huge dungeon with a nasty boss at the end, and let me tell you how scared I was taking on these hard skeletons as a level 1.