I would like to visit the Black Marsh, not only because it's home of Argonians, the strangest race of all, but it would also imply a different style of gameplay and story. Slower paced like Morrowind, with diseases, Hist and other natural beings that make the land harder to understand and explore.
These game features could work well in a harsh territory like Black Marsh:
- darker nights, more horror elements (creatures, plants, posions) that would make the adventurer less safer outside the beaten track than in Oblivion or Skyrim
- hardcoe mode (optional of course like Fallout's) that takes natural hazards into consideration and not all the wounds and diseases would be cured in traditional ways, there would be a shaman/healer questline to access the elders who can help you discover the secrets and remedies of the swamp (ecology/shaman/druid-based faction)
- sailing skill, sailing boats in real time
- riding wild creatures of the swamps, the horses aren't efficient in the inner land; maybe amphibious mounts that can be used on water too
- all the races and variations of Argonians ; not necessarily playable but they should be all depicted in detail with different life, settlements, rituals, etc
- underwater points of interest like sunken ships, old ruins, tunnels build by mysterious creatures; the adventurers will be more rewarded the more they dive deeper into the wet heart of the province; there would be all sorts of underwater predators and the player can fight them back under the water
Other game suggestions that I'd like to see return (or implemented) are:
- people naturally react to bad weather (and comment about it), like The Witcher
- wearing clothes and armor (Morrowind style)
- swimming, climbing and riding skills
- open cities that load the cells progressively as you approach (no city gate)
- locational damage for more engaging combat situations, npcs reacting differently to different body parts damage
- lockpicking is real time, you gotta watch your back while tresspassing
- more diplomatic quests that don't require combat, more ways to do the same quest