1: Reputations. This was one dynamic lost since Morrowind that I feel really added an extra layer of depth to simple questing.
2: Several factions. Not just the typical, Fighter, Mage, Theif, Assassin guilds. A local rebel force, seperate religious institutions, maybe an 'dark' mages guild.
3: Custom Spells. This irked me so much when I read about Skyrim. Sure they did alright countering it, but in the process they took out spell types, and severely reduced complexity in enchanting.
4: Store owners who are actually able to buy the stuff I find without perks, or needing to wait through weeks or months of in game time to empty my inventory from a single run. Make tiered shops or tie it into reputation, but don't make me clog my inventory with soul gems just so they have enough gold to buy an enchanted ebony sword I found.
5: More armor slots, seperate hands, feet, shoulders, at least 2 rings (3 if argonian or kahjit cause they have tails). I miss the glass gauntlet Pimp Hand I had in morrowind. This would also allow for increased enchanting complexity.
6: No more table alchemy and enchanting, or at least ways to do it without a table. Maybe not as powerful, or some other restriction. It was nice to be able to gear up and go on a potion run, and just clear the landscape of ingredients while making potions on the go to clear up inventory space.
7: Adjustable enchants, high power single use, or low power multi use. If I want to make a weapon to blow a who grand soul gem in a single stroke, let me please.
8: Armor and weapon damage. At least armor dmg if not weapons. After you reach your max armor in Skyrim, there's really no need to touch the blacksmith again.
9: Arrow crafting and enchanting. Nothing is more irritating that having to carry around 2 or 3 different bows for different effects. This is especially poinient if some of the bows are one offs like the Dawnguard bow you shoot the sun with.
10: Property escalation. Small house, to medium residence, to large manor, to external castle. And a couple of options in each catagory would be nice. Instead of the straight incline we see today. Maybe 4 small house choices, 3 medium residences, 2 manors and maybe 3 castle choices. Each of these could be spread amonst the towns so that maybe one town only has 2 small houses, another has 1 small and 2 medium, another has 1 medium and one large and another has only 1 large.
11: Different options for decorating said houses. Maybe in one small house I want to be able to craft a lot of things, so I'll need crafting things and lots of storage, while another I want to use as a show off house, so I'll need lots of weapon/armor racks and showcases, while in another I may want it to be a library of sorts so lots of bookshelfs. This way having multiple houses can actually serve a purpose instead of them just being coin sinks.
12: An actual pack mule companion. Maybe I'm so bad ass I don't need someone to fight with me, but being that bad ass, has cost me the ability to carry much more than a single warhammer out. Would be nice to have something I could park in front of a dungeon that I could step out and dump everything on, and that would have massive carrying capacity (especially for moving houses).
13: Auto run on horses, and multiple fast travel options. Boats to some villages, Carriages to others, maybe even occational groups of NPCs that setout from unconnected/connected settlements, and travel to connected/unconnected settlements. So when you're low level and can't afford the boat you can get with a group (or put one together) to travel to the next nearest city.
14: Replaceable semi-essential characters. People like shop owners or quest givers. If one of them dies, they should be replaced by a random NPC that acts like say a cousin (quest) or a new owner (shop). This way you don't have to worry as much about large battles around or in villages. I'm always terrified when some event happens in a city and people start fighting. I never know if I'm going to have someone to sell stuff to after the battles over, but I don't like the idea of having completely immortal NPC's walking around. Even if I had to wait a while it would be nice to not have that fear.
15: Some static loot. Some things like the deadric artifacts should be powerful through-out the game. I shouldn't have to wait till 46+ to pick up chill rend because it's damage will be gimped if I don't. On the converse obtaining these items should require a certain level or proficiency to obtain. Chillrend should have the same damage if you get it at level 1 as it does if you get it at or after 46. But it should be so difficult to get that only a 46+ person should be able to get it.
16: Sneak and invisibility should be less game bypassing. Bosses, and maybe lieutenants should be able to see through most (100% for one 90% the other) sneaking, and magical type bosses should be able to see past invisibility. Making better traps would help a lot with this.
17: Plateauing potion and enchanting effects, so as to eliminate stacking "God" buffs. So that to become immune to something or to cast for free you would need fully enchanted armor/racial perk and some potions.
18: Better planning with weapons and armor for expansion. So that the armor you get in expansions isn't out shined by the armor in the original game. This basically means, planning the leveling curve to include future expansions rather than basing it off the vanilla framework.
19: A storage option in the beginning of the game. Maybe some form of bought public storage area. Like a shop that sells storage spaces. Cause there's a lot of things you get in the early game that some people would like to keep (special armors) but can't due to low inventory space and a lack of home at the beginning of the game.
20: And Finally: More planning and expansion on the late game. The last 2 installments have changed significantly with the addition of leveled mobs. The game has become where you can run any dungeon at any level with success. This leads to people either simply sneaking all the way through, or just barreling through to get the goods at the end. If there were certain dungeons that were leveled and others that were adjustable it would make players question repeating what they just died to, rather than simply blunt forcing or using a gimmick just to get through. Things like the deadric artifacts should be end game quests, not things you can do straight out of Riverwood. They any many a side quest should be things you GET to do after you've helped people around town, or reached a high enough level to speak with whatever deadra.