You're making my mouth water.
I would love for some big bad to wander in, and try to stop me. Imagine how epic the fight would get! I'm imagining the first time I tried to kill Zixzax in DOS 1. It was an epic fight, and it took me like ten full minutes just to beat him. Too bad there's no implications for doing so; he just resets. But the thrill was unreal! Actually doing battle with someone powerful enough to put up a fight was exhilarating. I'd love it if Ye Olde SWAT was called in to go ham on me; I'd love the chance to fight through them, maybe get some sweet loot, maybe a smidgen of XP. Or even not. Just the achievement would be oh so sweet.
I can see it now; standing over the corpse of Griff, knife in hand, covered in his blood, when the Magister Armaments Division (MAD) drops in with teleports, yelling at me to cease and desist. I cloak and dagger, rage, sneak, vault, yada yada until they're all on the floor, and I take a severed head and scream at the heavens in a rage-induced challenge... And the boss drops in.

That sounds so epic, I want it to happen now.