I've got a really good one. Well, in my opinion I've got a really good one.
Since Nords are based on Vikings (well I think they are), and we see the presence of the East Empire Trading Company in Skyrim... You see what I'm getting at!!!
PIRATE/VIKING expansion. We've already seen Daedric planes and there's not a lot of water content in Skyrim. Just a few things I'd love to see in such an expansion:
1. Moving ships. Possibly cannons on those ships (Dwemer cannons, anyone?) Possibly sinkable/destructable ships. I have a feeling the engine won't allow for any of this, since they can't even get ladders to work.
2. Epic Water Monsters... Sea Dragons. Krakens. Leviathans. War Turtles. Giant..."insert water creature here." Peferably scripted single time bosses so it's not so loose in structure and it remains an epic memory 10 years from now rather than annoyance you want to forget, i.e. how dragons should have been.
3. Since Skyrim's in the North of Tamerial, we could always see an invasion of some sorts from a new race, from a different continent. Or scrap the invasion aspect. I'm sick of invasions. I just want to RP! Maybe a distant relative of the Argonians. But rather than evolving in swamp land they evolved in/near the ocean.
4. Water element (magic). Summon forth a dragon made of water straight from the sea. Tidel wave Dragon shout. Turn water into ice for a short period... Water based weapons and possibly armour. That could be a side quest; collect a full set of legendary water gear.
5. Obviously you'd want Nords and the East Empire Trading company to have some form of presence in the expansion. These particular Nords should probably be a little more barbaric and isolated from Skyrim's typical Nords. I want to see some pirate action. This could be one of several 'main questlines'. Pick your faction, baby! With the mention of more barbaric Nords, give us a new armour/weapon set based on Vikings but a little more... exotic. I'm also with the idea of Aedric armour and weapons.
6. Along with icebergs, I'd love to see some sort of 'Waterworld' towns (google the movie), floating in the ocean. Possibly some underwater towns (i.e. see Star Wars).
7. Flyable Dragons. I understand why they might not have been included in the original game. But with something like this, I see no excuse... Well, unless the engine wouldn't handle it.
I have more but that'll do for now.
EDIT: I'll just leave these here. (Images)