1- spellcrafting, obviously. Wait; Too spreadsheety? Change the way spells are crafted then.
2- Attributes. Skyrim lacked attributes, and enchantments on items became these horrible things that offered numerical values on highly specific things. Like 'this school of magic costs 20% less' or 'one handed and two handed attacks (but not unarmed?) do 10% more damage. It was gross and cluncky. Now I'm not saying that Attributes in the past games were perfect (trying to get multipliers for them was pretty aweful) and frankly I'd rather a system where they get increased as you work, or something that's mostly static like fallout's S.P.E.C.I.A.L that you pick at character creation and don't improve much.
3- Advantages and disadvantages.
Most players haven't gone that far back in the series, but daggerfall had a ton of optional merits/flaws that you could pick up during character creation. They ranged from phobias of certain enemies, to taking damage on sacred ground. They included some excellent options such as where the player struggles to use magic In either darkness or light, or that they recover faster when in darkness,light, or submerged in water. There were things like 'forbidden materials' that you couldn't use, and weaknesses to certain things. These helped to build the world and altered the playstyle of the player. They would have also changed the way we could fight npcs were they to have these same advantages/disadvantages and the AI to use them.
4- Layers upon layers of swag.
So In Every TES game we've had less options in what we've worn than the last, Mostly because as graphics have gotten better and better, clipping has become worse. In skyrim you have an amulet, two rings (or was it one now?) boots, gauntlets,helmet and then 'armour'. In morrowind you had boots,left gauntlet, right gauntlet, trousers, greaves, skirt, shirt, belt,two rings, cuirass, amulet, Left pauldron, Right pauldron, helmet and robe (yes, you could wear a big wizard robe over your full plate) In daggerfall, I think you had more. At least cloaks.
Now. I'm not saying it was perfect (Keep those shoulders together!) But, it was boss, even if your customisation was ruined by the fact that you were too heavy and needed to wear a stupid purple robe over everything to keep yourself afloat. But, apart from being cool as cattle to be able to have this degree of customisation; All this stuff was neccessary, because enchantments were much cooler
5- Enchantments that were awesome.
Right, so you used to be able to enchant things so that they were essentially re-usable scrolls. You could have a belt that summons scamps, or a helmet that replaces itself with a bound helmet whilst simultaniously conjuring a dremora. There was a pair of boots that would make you move superfast at the cost of blinding you whilst they were worn. Apparently a shield that was scripted to kill vampires who used it.
Also, Enchanted items recharged slowly on their own. That was better. I miss that.
6- Mountains of missing spells.
Now, the most famous one being levitate (Because who doesn't want to fly?) But we also had things like locking doors so that things wouldn't chase us, poison spells, CHARM; The instant friender. swift swiming, water walking, chameleon, sound, silence (which sadly shut nobody up, but instead stopped them casting spells)
Also, The schools of magic that spells were put in were wrong in skyrim. Not arguing a case for bringing back mysticism, but rather things like paralysis and light not being in illusion. I can see the function of paralysis in alteration (more temporary petrification) so I'd not be opposed to having Both, but no cuts when you paste, please.
7- Acrobatics and climbing
So In the past few games that had acrobatics it wasn't much more than jumping high and falling slow (which admitedly got us to jump over a few enemies a couple of times) but, with the awesome parkour elements of games like assassins creed, mirrors edge, dying light, overgrowth or what have you.. the point is; We have the technology. Yeah, sure, you should be able to circumvent this kind of stuff with flight, superjump or some kind of enchantment (all of which are reasonably hard to get) but it's choice. It's cheaper, and offers the lightly armoured rogue more opportunities for positioning, chasing,escape and alternate routes. (also, if the level designers want to be cruel, they can 'enchant' and area into a no-fly zone)
8- Little dances and strange noises. Magic.
There was something so happy about casting a spell in morrowind. You'd do some hand gestures, use both hands and had a chance of failing if the spell was too hard (rather than, say, being totally barred until you hit a pre-set milestone, which kinda svcks) Morrowind would give you a light show, and then you had tutors to teach you the spells, most teachers teaching you their own variation of the spell. It was a lot more meaningful than simply throwing an arm forward (repeatedly) in Oblivion and a lot more magical than 'bioshock hands you got from books' in skyrim.
10- Spears, unarmed combat and throwing.
We have the technology to make polearms, the most common and widely used weapon class in history, and unarmed, the most widely used type of combat that doesn't involve weaponry (That was a joke, Laugh.) But seriously; We have the technology to make them both look good, and lore that says monks win. If you're going to want to fight some big monster, you're going to want the reach of a spear. If... well, punching,kicking and throwdowns have always been satisfying. Based on the level of power people can build themselves up to in Tamriel, I fully expect to have a dremora put a fist through my heart in the next encounter
11- Different ways to use your weapon.
Slash slash, Stab stab, chop chop, bludgeon O'bludgeon.
Games before Oblivion had different damage values for different movement, but seriously, it would add a lot more skill to combat if we had to... well, use actual skill. In real life, a man would change his style of swordsmanship if he was fighting a man in plate, because slashing with a sword isn't going to bother a man much in plate. You must poke him. Similarly, we've got some bad ass undead fellas who're probably going to throttle you if you stab them. You need to slice them to pieces. It's all in good fun. Also some argument about reach and how many enemies you can deter at once.
12- Boobs.
Nope. Totaly serious. Daggerfall (low pixel) boobs. Actualy, daggerfall had quite a lot of scantily clad women (most of which wore hoods, for modesty). But there's really something less terrifying about a daedra who cares for modesty. Make me think my character is in for a good time, seeing a naked beauty in the bath.. then reveal why the call them 'spider daedra'. Also, I need to see a nvde hagraven.
But yeah, why not throw in the sixy character who misleads the player into thinking He/She is up for a good time, then throw a wrench in and have it a monster. You can do it tastefully. I have faith! (please, for the love of colovian fur, don't leave it to the modders)
13- Of the same style, better material.
So in the later games you'd have this trend of the better weapons looking far more exotic (for better or worse). In daggerfall? You could have a sword that's the exact design and shape, only made of a different metal, making it a different colour. Same thing for armour. (jewelry doing this for most games. Though much more simplified in skyrim)
Also, Resist normal weapons. Bring that back. Maybe expand on it. It'd be interesting to have an enemy that's weakness is specificaly mithril/ebony/poisons with bloodgrass in/ wood/gold/clay urns that can be thrown at them.
Though, if you're going to bring glass back, please use morrowind's style for it. Also; No corrundumn/quicksilver/machalite/moonstone/ using smithing to make daedric (That's a ritual, not a blacksmithing practice!) Would be happy to have adamantine back.
14- Talks with commoners.
So, back before voice acting, you could find out about everything from anyone. Locations, lore, quest info, what something was. Then we had voice acting, and we get lines like 'do you get to the cloud district very often, oh, what am I saying? Of course you don't.
15- Cunning traps.
No, Obvious pressure plates are bad. I'm talking about tripwires, enchanted doors and chests, pressure plates that are hard to see which don't have an obvious death dealer looming by.
17- Flying enemies, and also; Floating, skeletal ghosts. Please stop using normal people who're translucent and glowing. (though that house in skyrim with the bal shrine in with the poltergeist stuff? more of that. )
18- morrowind dunmer voice (also, the game 'redguard' had this voice. Jiub in dawnguard). It made the dunmer badass, even the ones who weren't. That, combined with their swagger, swept back ears, and entirely red eyes gave them ultimate badassery (though the netch leather, japanese weaponry, tongs and good architectural sensibilities certainly helped) Come oblivion where they have the horrid voice of the elves, and skyrim where they have the same voices as everyone else and you've got a sure downgrade. Please go back to where it was good.
19- Talking with monsters.
Games before morrowind had the language skills. Now, I don't really care for leveling up a skill like that (though I'm happy with other ways to learn languages. Maybe a collection of books that you can practice with during your long wait times.) But in a rollplaying game, it's never bad to be able to play the guy who never draws his sword. (and even better to play the guy who talks the enemy out of attacking him, and then attack)
20- Daedra, Daedra everywhere. There are so many of these fellas that don't make it into the next game, despite them being cool as cucumbers (that rip hearts out). Spider daedra, scamps, clanfears, daedroth, winged twilights, a bunch of guys from battlespire, an ogrim. Maybe one day an aquatic daedra.
21- Things under water.
Swiming, hitting things when you're swiming. Diving for treasure, getting the hell out of the water because there's a dreugh coming. We'll have more of that, please.
22- Decent guilds. I miss those. Fighters guild, mages guild, the morag tong, the dark brotherhood when it was fun... yep. Those were the times.
23- The ability to soul trap anything, soul gems remembering the name of the thing they've trapped, souls having their own numerical value (we don't have to be show this) and items having a different enchant limit depending on it's quality.
23- the ability to kill everything that lives, even if it's a god.
25- The ability to steal everything. Absolutely everything. No hidden chests that are invisible under the map; Steal everything that a shopkeeper can offer, steal a man's clothes as he sleeps.
26- Skills that make the skills better, rather than not making the skill better but instead allowing you to buy a perk which makes the skill better.
(if you want to do perks, don't do them like that)
Petty things like: Vampires and Dremora turning to ash and having similar though still absolutely terrifying voices . Vampire clans with actual clan identity. Alternative werecreatures. The fork, you know the one. alcohols. Some variety of samurai swords. a main villain with tons of lore who turns out to not be just a generic black dragon. Dwemer living in metal halls. strange alcohols and mountains of moon sugar. Annual Holidays. the chosen one status shoved into your face repeatedly. The ability to place a bucket over someone's head. Bows not being made out of metal. Horsearmour (that isn't DLC) crossbows (that aren't DLC) Alchemist recepies, the ability to carry tools with you (alchemy,enchanting,armourer) and use them whenever you want.