Yeah, more spells would be great again, too. Morrowind and even Oblivion had a ton more spells than Skyrim. I'd especially like to again get levitation, spells to unlock things and more diverse conjuration spells - I mean in Skyrim you can basically only reanimate corpses or summon Atronachs with actual spells (as far as I remember). I'm missing the Skamps, Clannfears and Daedroth.
Also it would be great to be actually able to play as a mage without being forced to use destruction magic. I installed a mod back when I could still play Skyrim that added a ton of new spells, amongst them spells that do damage of every magic school - that was really nice. The mod also included spells with which you could raise a wall in front of you, something with which you could create drop zones in which you don't take fall damage, a restoration spell that upon touch instantly killed an enemy if it was below 25% health (made my life so much easier with the Dragon Combat Overhaul mod installed...) and other stuff like this.
So yeah, instead of simple variations of the same handful of spells it would be great to have actual diversity. Also it would be nice if you could create your own spells again.
I also installed a mod (Simple Magic Overhaul by ApolloDown - he took it down, though) that changed how magic worked, but never could try it out since my computer began having problems with running Skyrim back then already...
From the mod's description these seem to be the basic contents:
a) Depending on your race you start with 0 to -100 Magicka. Not everyone can be a mage. Probably not something I'd like to see in a vanilla version, but for modded playthroughs that's at least interesting. By reaching level 65 in a spell school you get permanent +50 magicka, though.
b) Something along the way of the more magicka a spell uses up the more powerful it is. If you drain your whole magicka with a single spell you do like 3 times the damage the spell would usually do
c) Still you have to be careful, as your magicka stops regenerating if low enough + if you run out of magicka through a spell you apparently can experience knockback that can actually kill you
d) Low stamina reduces the power of your spells by up to 50%
e) In "desperate" situations you have a chance of randomly firing off a vanilla spell of any level
f) Attack power of your spells increases with the level you have in that school. Apparently combined with b) your spells can thus reach up to 9 times their vanilla power
g) Wards, if timed correctly, can reflect spells - making them actually useful
h) You can deal damage with telekinesis by throwing stuff at people.
I don't know what you think about this, but I think this sounds awesome.
So, thought a bit more about the weapon types/classes I'd ideally want to see in an Elder Scrolls VI game:
1) Short weapons:
Knives, daggers, tantos, wakizashis, shortswords.
Should be pretty effective when used to stab and also be especially effective while sneaking. Shouldn't be very useful against armored foes except if used to stab.
Cutlasses, arming swords, rapiers, longswords, katanas, greatswords.
Most noticeably here I'd return to divide weapons by what kind of weapons they are, not by if they're onehanded or twohanded - after all using a greatsword is more alike to wielding a longsword than wielding a warhammer.
Also you should be able to choose if you want to use your katana or longsword onehanded or twohanded - onehanded your attacks would be slower and weaker, but you could use a shield or magic with the off hand. Twohanded your attacks would be faster and stronger and parries with the weapon more effective.
Katanas would be most effective against unarmored or lightly armored foes, as would be rapiers. While you'd slash with katanas, you'd stab with rapiers for the most damage.
Cutlasses are slightly better against unarmored foes whereas arming swords are a bit more effective against armored foes.
Longswords have a higher reach and higher damage output than arming swords and greatswords even more so.
3) Axes and (most) blunt weapons:
Onehanded axes, twohanded axes, maces/clubs, warhammers (onehanded and twohanded)
I'd combine these two since from what I've seen there doesn't seem to be a big difference in how you use them.
All of these weapons do most damage by swinging them, whereas they're generally rather useless for stabbing. Axes include throwing axes which you can also use in close combat. For throwing them to the most effect you need a different skill (6)), though.
All of these are also particularly effective against armor compared to other weapontypes and the twohanded variants do very high damage.
Downsides are that these weapons are heavy, taking up a big part of your carry weight, use up stamina a lot faster than most other weapons and while they're not much slower than, say, swords when you simply swing them, it does take a great deal more time to recover from when you land a hit. Also blocking with these weapons isn't very efficient (after all your fingers aren't protected).
4) Spears, polearms and battle staffs:
Spears, poleaxes, halberds, battle staffs
All of these have a decent to very high reach compared to other melee weapons and have to be wielded twohanded. Spears are only good for stabbing and it's pretty slow and ineffective to do aything different with them. Halberds are much the same, but can do high damage with swings, too - still you're incredibly slow to recover from such swings. These two weapons have the highest reach of all melee weapons. Especially halberds are pretty heavy, though. Spears include throwing spears that can also be used in melee combat. Your skill in throwing them depends on a different skill (6)), though.
Poleaxes only have medium reach but are good in pretty much every aspect against all kinds of foes. While tehy don't reach the power of a twohanded axe or warhammer, they are a little lighter and faster.
Battle staffs don't do much damage, but since they'd be enchanted most of the time their use as a melee weapon only is a secondary thing anyways.
5) Bows and stuff:
Bows, crossbows, maybe blowguns?
Bows are the quickest to reload, but are not that effective against heavy armor. Crossbows do the most damage and aren't bothered by armor (maybe ignore even part of it? - wasn't that already teh case in Skyrim?). Blowguns do nearly no damage and are closeto useless against completely armored up foes, but they're stealthy and their darts can be poisoned to the most effect out of all weapons (?). Maybe there could also be special alchemical/magical projectiles that explode or something.
And you should be able to use arrows as stabbing weapons (short weapons?) if you need to, although they're really not very effective and break easily.
6) Other ranged weapons:
Throwing knives, throwing darts, shurikens, throwing axes, throwing spears.
You should be able to use all of these in close combat, too - to various degrees of effectiveness. Knives, darts and shurikens are really only viable against unarmored foes - they're mostly for fun, anyway, but I guess if poisoned or enchanted they might actually be useful. Throwing axes and throwing spears do quite a lot of damage if they hit, definitely being able to penetrate armor that isn't plate as well as lighter shields. If blocked by a shield that isn't "good enough" they have a chance of getting stuck in it, encumbering the opponent, making his shield blocks slower and cost him more stamina when blocking and moving.
Did I forget anything?