Some things that I think TES 6 could benefit from are:
The return of classes and skills such as unarmed.
-Needs no explanation, this so called "streamlining" is getting rid of a lot of replayability in my opinion.
An improved melee combat system that allows for parrying with weapons and shields for a more fluid combat experience.
-One way I thought that this could function would involve timing attacks. For example, if an enemy and the player attack simultaneously the character with a higher weapon skill has a higher chance of parrying the attack, leaving the other open for a riposte. This would also make it possible to defend yourself when dual wielding weapons.
-This would also require better AI that don't spam attacks and then randomly block for no reason.
-If AI could also circle the player and wait for an opening to attack, instead of running straight in with no thought for self preservation that would also be cool.
Quick cast defensive spells that allow characters to defend from attacks.
-Some suggestions for this could be a spell that needs precise timing, but if used correctly can deflect an incoming spell. Against melee users, the ability to push back an enemy to open them up for an attack would help for melee defence.
Improved stealth
-Enemies don't forget you if detected and will constantly be in an alert stage once they know something's up (Their friend gets shot with a bow and they actively look for you).
-Being able to pin yourself against cover and peek round.
-Audio cues when you get spotted, even if that's just an enemy shouting that they found you.
More mobility and verticality in area design?
-In my opinion this isn't that important but it would be cool to be able to climb structures to some extent.
A guild progression system like that in Morrowind
-In Morrowind you actually had to be competent at your craft to progress through the ranks of a guild, nuff said.
A more lifelike world
-People acknowledge the state of the world after events.
-Dynamic NPC inventories: clothes change, inventory is affected by what an npc does and their schedule (If an npc is on their way to the market, they have more gold on them. After they purchase goods, these can then be stolen by the PC from their inventory).
-Random events happen to NPCs such as going to other cities on errands(In the case of shopkeepers, their apprentice will take over for the time that the shop owner is gone)
-People actually change in wealth depending on in game events
-Actual town size towns instead of hamlets being called cities.
-Random NPCs that wander around such as mercenaries that take jobs, then will actually go to a location and carry the job out.
-Disposition so that people react accordingly to what you do to them.
Character choices and roleplaying
-The ability to be the bad guy and start/join a bandit gang.
-Choices that matter, such as having to make difficult decisions that result in different outcomes.(Something akin to The Witcher or Mass Effect here).
All of these are my own ideas and are my personal opinion on what should be in TES 6.
-Edit-
Some things I forgot to add are:
-The possibility of exploring skills like magic further than what is known by ingame NPCs to make it seem as if the player character is actually leaving a legacy by discovering things. Sort of post-mastery skill progression.
-More unique items. It might just be me, but collecting all of the unique items was a part of the game I thoroughly enjoyed, I feel as if in fallout 4 this has been detracted from in the form of legendary items which are essentially just generic items that have a special ability. Make these items look and sound unique too.
-Another dig at fallout 4 - more interesting locations to explore. The only problem I had with this in fallout 4 was that in between the interesting areas, there is a wealth of boring and generic locations seemingly put into the world for the purpose of gathering crafting materials. This needs to be addressed in TES 6
-Weapon and armor modifications. Something that was really well done in fallout 4 was the crafting, a similar system for creating new gear in the next TES would be awesome.