I prefer the system of Morrowind, It is more realistic and better than those of the last two deliveries (Oblivion and Skyrim). I hope that it is in Argonia or Elsweyr + Valenwood with a system like that of Fallout: New Vegas, in which you had to eat, sleep and drink (In Skyrim it is possible to cook and eat, but it does not become not important at all by it). I hope that they do not return with the use of the shouts and dragons (Also I hope that the things are not simpler than they are now).
I do wish the system is much more realistic in the ways of movement and interactions. The animations were kinda weird and i felt that there's no real difference in the combat. The sleep and eating system would be very cool. Either way I'm more concerned about combat, armor, weapons and character creation. I feel that with a game that is pretty large and very popular there should be more of a immersive feel between the player and his character.
The NPC also they must put determination, since now they are too simple. The worse thing of everything is now you finish a guild without being the class that prevails in him. For example, you can finish Hibernalia's College to you being a warrior. In Morrowind you had to make other missions and you need a level adapted to rise of range. Now with two or three missions you are a major member. It would be good that they were putting the same skills that in Morrowind, as the acrobatics and the unarmed combat, are in the TES 6.
And... ¡MORE CREATURES!
Yes the more creatures because it gets boring fight the same two things, Draugr and bandits. I with they made a new way to level up skills like each skill is in a class on a side and you get a perk to invest on that side. Plus crafting skills should have there own section.
I like Morrowind, too, and I think it tells the best story and has the best lore in it (although Dragon Priests are pretty badass), but I find it much easier to roleplay in games as they come out and, I hate to say it, as more mods become available and the engines become more refined and easier to mod. The addition of proper food and a cooking system in Skyrim was a massive improvement, I think.
I'm most looking forward to the environment and world design because the last provinces left to do a game in are all very high-fantasy, like Morrowind was. We've had fantasy-Medieval-Rome and fantasy-Vikings, but something like desert-cats or tree-elves or the dreaded Thalmor (please no fantasy-Nazi-Germany) allow a more high fantasy game with crazy things (mushroom towers and stuff).
Expanding on the perk system would be something I'd be very interested in. I can't remember if Bethesda has a history with it, but I do know other game companies take tips from modding communities before making the next installment and I hope they look at things like PerMa or SPERG, to add more depth or individuality between skills.
Also, THROWING WEAPONS. I want a return of 'em. Darts, knives, axes, throwing stars, maybe potion or dwemer-inspired grenades.
I'm hoping it will be in Blackmarsh. However even if it's not. Please bring back spears... You know, long poles with a sharp pointy bit. Spears... Also much more imersive werewolf playability. I still think the companions questline was rushed. There was so much they could have done. Also with regards to werewolves. why couldn't they make it so you could join the silver hands. Not that I want to, but that would definatly add to it.
Also I would love for them to upgrade the followers so they act like proper NPC's rather than combat sheep who blindly follow you round. One other aspect I liked about Morrowind was the open all hours shop. Ok it adds realism but give us one shop please...Will it realy hurt?
Definitely agree with having some basic needs realism, and with next-gen (and powerful PCs) I'd like to see Black Marsh, Elsweyr/Valenwood and the Leyawiin area combined. It might be a stretch for Bethesda, but surely would be possible (and amazing).
As for new mechanics, maybe some sort of parkour. This could be general ledge-grabbing or a restricted skill set for thieves to clamber onto rooftops, which would open up the way for gadgets like rope darts or grappling hooks. Whichever, it would certainly be a boon in the sense that it would allow the player to tackle a dungeon in a less linear fashion, e.g. dropping through the chimney of a fort and infiltrating from within instead of bashing down the front door and progressing in a straight line as usual.
Also, better dungeons. I understand that there's so many, but if each dungeon crawl was a memorable experience then we'd despise each "fetch this" radiant mission from nearly every quest-giver much less - think Frostflow Lighthouse, where one journal gets you interested in finding and avenging the family by delving into the chasm below (you even get an active effect for respectfully burning the remains). This could be combined with much better A.I.s who hunt your last known location, take cover and actively cooperate with allies, or acting like the stalkers in Assassin's Creed Rogue.
Finally, a cover system could make or break the game, since it would ensure you could avoid being hit but also be dysfunctional whenever you need it (*cough* AC Unity *cough*). Sorry for the long comment, just some ideas I've had for a while. I'll still enjoy the next game guaranteed, even if none of these are added.