I actually just want to see them extremely hard to kill. Like, if they were actually the biggest bosses in the game. And it would take a crazy amount of people to kill them.
The hilarity of trying to pull that off in some city would be, well, hilarious.
All Elder Scrolls games have allowed us to kill guards. Most MMOs allow guards to be killed. So why should we not be able to kill them? They system in Skyrim was fine with there just being endless amounts of mortal guards. Killing them opened a way for you to escape arrest. A similar system would be enjoyable, but I'll admit it wouldn't really change my view of the game any in either case.