It would be for the best if they just modified the world they have already created. Add some new stuff to skyrim. Maybe make the DLC activate only after certain events have transpired in game. A Thalmer invasion, a Forswarn Insurgency possibly even 'the Draugr walk' a zombie spin off
My point is if they just added to the world they already have, they could spend less time on world creation and more time on deep characters/plots/quests with a real impact on the world around you. The problem with Skyrim is that they haven't really added the whole 'real impact on the world around you' part to the main quest. And if they are going to do a DLC they should fix this, or add another questline which does.
On another note that needs fixing:
It seems the easiest way to fix the Dragon Invasion is to avoid it all together. Maybe if they do an Invasion of sorts make it so if the player does not act it continually gets worse leading to the destruction of the world around that. Would of been cool if they had made it that if you completely ignored the dragons for too long cities would fall under attack and people would have to form a resistance movement or be put to slaves for the dragons ala old time. It seems that if they did set the game in the old times of dragon oppression it would of been more interesting.
Maybe that would be a good expansion pack, fight the dragons or they take over the world, not just hunt mudcrabs.