I know I'm looking too far ahead, but how does Bethesda top the challenges in Skyrim?
All the games had something special...
In
Morrowind you:
(1) killed one (or more)
living gods while
(2) becoming a near immortal. (You stopped aging and became immune to all diseases)
In
Oblivion you
(1) fought a god (or more) while
(2) closing gateways to another (demonic [looking]) realm
and could even
(3)
become a godI'm excited about a new ES game coming out but somehow dragons, one more god and people with dangerous voices don't top becoming a god or a near immortal. They might rank somewhere around there (depending on how it's going to be implemented) but not above.

I think it all comes down to creating an immersive world where you do a lot of great things not one HUGE, EPIC thing. ...It wouldn't hurt to add the Tsaesci (golden skinned snake people of Akavir) to the game (that would top it for me)