Make water more dangerous than Oblivion, but a little less so than Morrowind (something should NOT attack every 50 feet, but it should be more than a mudcrab every 15 minutes).
Give us a reason to explore the waters. If levitation returns, give us a lot of reasons to learn it, and some subtle hints about where we might want to use it. Falling rocks/dirt, dripping water, dust. Whatever it is that says to someone exploring "by the way, look up!" but doesn't scream it.
Offer history where the lore is specific. If you're doing Skyrim, Solitude had better have a library that in some way has a place someone can live under it. Sewers, front steps, basemant? The lore isn't clear. But somewhere in that city, there is a library that someone lived under. Likewise, there is somewhere a young scion of the Septims skinned a cat during a thunderstorm, thus ensuring his place in history
Pooint is: I should be able to plant my character somewhere and know that HERE is where it happened. It could be the middle of a street, behind the barracks in the castle, or buried beneath the counter in a bar. But it's quite obvious in the lore that IT HAPPENED, and we should be able to pinpoint the spot... with effort.
So what if it's not Skyrim? Well, check every in-game book ever written and see which ones apply. If somewhere is a battlefield, I want to find broken/rusted armor, skeletons, ashes from the funeral pyres. Something.
Local gods. If we're in Hammerfell, I want to see Yokudan gods having temples... and those temples having quests and content. By content, I mean Knights of the Nine type stuff where I feel like I'm serving a *god*, not a priest. Morrowind's Temple had a bit of a good thing going (pilgrimages, recovering relics), but it still doesn't feel like I'm serving the diety.
New books. I'd hate to play THREE games in a row where the rare and funny books are all stuff I've already read. twice.
The ability to filter spells and alchemy fodder by effect.
Nerf the fortify attribute/skill spells (just making anything more than 20 points "very expensive to cast" would be a step towards them not being ridiculously powerful)
Move away from the every 25 level idea. Give me a reason to go from 30 to 31 in destruction, beyond "small cost reduction on destruction spells" and "well, that's one less level before we get new spells".
Multiple quests per daedric prince, and they aren't always coughing up the good stuff.
Sub-factions like the Bal Molagmer. Because it does feel special when you have requirements to even try the quests.
Always, Always, Always: new ways to interact with NPCs when talking. Suppose we bring back Daggerfall's Streetwise, etc: Pressing left/right on a console (or a/d on a default keyboard) shifts your speaking style. So you can politely ask the Countess if she knows where you might find her nephew, but you TELL that begger that you need to know everything he knows, or you'll rearrange his anatomy so that he kicks his own teeth out. From the inside out.
A magical device that can read a book (including the quest-related material from my journal) to me while I wander. Call it a "Modified Dwemer Parrot", or maybe it's a lesser Daedra bound into a form that makes it a useful familiar. This isn't exactly a major enhancement, but I noticed that it lets you experience FO3 while still "going about your business".