I want a underwater city like atlantis, and if you complete their quests you replace what ever the name of TES sea god is like you did for Sheogorath in SI. You could also unlike a trident as a new weapon, and the ability to summon krakens, sink ships, make hurricanes, Tsunamis the works
UGH if there going to due a DLC with a underwater city, they should not do a cliched greek mythology rip off, it should a black, "monolith" , slimy, and abyssal, thing from nightmare, with ancient horrors that cannot be slain, similar to Ryleah from The Cthulhu mythos.