Considering the fact that we now have a levelling system similar to Fallout's, which enemies would you like to see fulfil the role of the constantly high level enemies (an equivalent of the Deathclaw in Fallout:NV for those that have played)?
Of course there are dragons, but I'm thinking more along the lines of your average creatures.... From the list there's been announced already there are several potential candidates for higher level enemies (giants, frost trolls , big spidery thing....) but what would you personally like to see form Skyrim's tougher fauna?
We know already that not every creature you encounter will immediately attack you so should high level enemies always be hostile?
Daedra, Daedra, Daedra! I.E. particularly Dremora Lords, Golden Saint and Mazkan, Xivalai etc.
And, obviously, The Dragons.
Certain powerful Dragon Priests,
Giant Shaman and Chieftans maybe???
The occassional high powered sorcerer.
Ancient Vampires and Ancient Liches ( both rare )
Wraith Kings, perhaps.
Frost Monarchs (greater version of the Daedric Frost Atronach)
And NO I do not think they should all be hostile all the time. The undead Dragon Priests, maybe, but Daedra, Giants, even some Dragons perhaps, might not always be hostile depending on the circumstances and Player history.