Spoiler
You go to a version of valhala and you and three nordic heros with aweful voices banish alduin- he explodes but you do not absorb his soul and when you mention that he was not absorbed it is suggested that he will return. It's very anti climatic and poorly told)
Anyways- what if he returns as the literal world eater? A dragon with the head the size of a mountain descending from the sky about to engulf the throat of the world? Then the player must engage in a verbal battle with the colosal entity. (this being the end of a new quest line)
Lore wise, this would make more sense. Alduin is "the world eater" and i realy doubt that the small black dragon could eat the world, bring the endtime or do anything of the sort.
Perhaps this expansion could also
Add an additional questline to each faction- you gotta admit that most of these were shockingly bad
Rebuild helgen
Rebalance skills
Fix things
add new daedra and spells

