I'd also like to see more islands. If it's set in Skyrim, or any province with a coastline, I'd love to see islands. Large islands, small islands, even islands a few feet wide. It would make it more worthwhile to explore the coast. I was so disappointed when playing Oblivion when I saw that there were no islands in the Topal Sea, and that they didn't add Stirk to the game.
I think it should, if implemented, have degrees of complexity depending on how high your skill is. For example, at Novice you can only climb things like rocks, at Journeyman you can climb brick walls, and at Expert you can climb smooth surfaces like the sides of a house and walls.
When I started playing Morrowind back in whatever year it was I started playing it, the first thing I did was to create myself as an Argonian because I could breathe underwater, then when I started exploring under water I found pearls in shells and I went further and found these big things that could kill me pretty quickly (dreugh) and I even found what looked like an underwater city (Vivec). Then as I continued to play the game I discovered very quickly that there wasn't much else underwater, there was a nice set of arches to the northeast of the main island but there wasn't very much down there.
I would definitely like to have a lot more under water variety, life and architecture. Not just a ruin here and there, not just a fully fledged city, but a whole civilization of different creatures down there. Mermaids, Dreugh, ???, ???, ??? and ??? would be nice, each with their own city, civilization, ideas, concepts, art, architecture, dungeons, caves, castles, technology, etc. Hopefully this has been implemented in TESV, if not, please make TESVI with this idea in mind. Empire of the seas.
I'd also like it if, when you are standing in a pool of water and you send a shock spell out at a mudcrab or a dreugh, that you get 30% of the shock damage transferred through the water.
Stephen.