As I stated in another thread, I would love to visit the Castles of Glass and Coral of the Dreughs.
Since I am lazy, I copy and paste here part of what I wrote somewhere else :
I think that the Sea of Ghosts could be a great place for new content. I would love to be given the opportunity to use boats to travel on it. New environmental dangers could be encountered : wrecking storms, wrecking icebergs, fast-growing brinicles, hazardous reefs, maelstroms, underwater volcanoes, ...
Among the new places that can be added with a DLC, there are Roscrea, Solstheim and the Castles of Glass and Coral of the Dreughs.
Dreughs have always fascinated me. With the withdrawal of Vivec's power, one can think a major impediment to the recovery of their lost civilization is now gone. Obviously, a recovery of their lost power could have heavy consequences on shipping, and thus on trade and politics. No doubt it could be used to build a nice storyline. They could be a threat, as well as they could be interested in tying up commercial agreement to sell their own stuff.
Many new creatures could be added through such a DLC, among which underwater Dragons, electric slaughterfish, narwhals, krakens, Lamiae, Medusae, Dreughs, ...
Among these new creatures, many are of the NPC type, (in fact, the ones I wrote with a capital letter), which means that they could have their own society, hierarchy, culture, religion, architecture, arts and crafts, leading hence to new lore pieces, new clothing and armor sets, new weapons, new artifacts, possibly even new spells.
Among the new creatures added, one could also count the clockwork dreughs, which are amphibious counterparts to the Dwemer war machines built by Sotha Sil when he lived in the Castles of Glass and Coral, and weresharks.
New "ores" could also be added, for example mother-of-pearl, cold-water coral.
Many new factions could be added through this DLC, each with its questline, clothing, armor and weapon set and its historical background. Among those, various fleets (military ones, but also pirats or traders's), some Seafarer's guild sponsored by Haafingar,
Moreover, since Sotha Sil once lived in the Castles of Glass and Coral, is it totally crazy to assume some remnants of the Sotha House, thought to be destroyed, could have survived underwater ?