I was thinking that the next DLC could be set in the Sea of Ghosts, with a huge underwater section and caverns, with boats and being able to build a crew to go along with your (named yourself) boat. Maybe it could even be possible to engage in a bit of piracy or heroics on the high seas with sea battles and boarding, e.g leaping onto a flaming ship as your crew follows, the enemy captain preparing himself to fight you.
To expand on the underwater sections, there should be a new helmet or a new spell for those who cannot already breathe underwater and there should be the ability to leap onto islands and dive underwater to explore caverns and reefs.
Other suggestions
*Fishing
*Trade Monopoly/Empire
*East Empire Company Quests
*New Weapons (Creatable/Enchantable Staves?)
*New Armor (Shark Bone Armor, Horker Tusk Helmet)
*Ability to create houses (Hearthfire style) on the islands

Maormer.
We need Maormer.
Playable Maormer.
I've wanted this since half a day after buying Skyrim and getting into the lore.
I want to be a Maormer now.
Do it Bethesda.
For great justice.