TES is not Dnd, so I don't really see how that matters.
I don't know how Bethesda does AI and sneak attacks in the next game, so I really can't say how instant hit spell criticals would work.
I agree and disagree. TES is not D&D however, a great deal of what made this game was adopted from that game. In any case, there are some aspects that we must remember, for starters a lot of what Bethesda did when designing their games early on is take into account the die roll system, they did this to simulate things like hit and damage, but these elements just like sneak attacks and such succeeding were all put into the game to simulate things that couldn't be simulated in game, like your clumsiness and inability to swing straight in early levels, your opponents agility and thus ability to dodge attacks, or in the case of inability to sneak attack with magic, the longtime understanding in most RPG's as based off the D&D and most D20 systems, that magic users use vocal, gestural, and physical components which give away their ability to do something without being noticed easily.
That being said, there's a great deal in this post I would like to address, but that's the first thing I would like to see. One of the complaints I here a lot about is the psychic AI, and I agree I want it to disappear, but in response, I'd also like to see a more accurate magic system, much more robust and creative, with a lot more options, and potentially more power, but a great deal more weaknesses as well.
Now before I get any complaints that the TES series does not take into account things like vocal components, or gestures for spellcasting, and that these additions would ruin the game explain to me why the spell 'Silence', makes a spellcaster incapable of casting a spell? Does the name of the spell have no meaning, or is it more likely that the developers knew precisely what they were doing, but the additional effort to put in the vocal components was too much of a hassle? Further why is it spells can't be cast when the caster is paralyzed if its all just a mental excersize? Once again another thing that makes no sense unless taking into account gestural components.
What I want to see is spells taking spellcasters time to cast, with a potential to fail casting if your gestural or vocal components are messed up, and I want to hear the vocal component and see the gestures, particularly at low levels. For Illusionists this will make their skill particularly difficult, but in that example, you see in D&D at least that illusionists tend to specialize in learning special talents to eliminate the need for both vocal and gestural components. Should an illusionist in the game acquire this ability, it should be reflected, and the spellcaster should be able to cast while paralyzed or silenced.
Further, components aren't only used merely to cast spells, they are also used to amplify spells, we've hear of wands, orbs, and various other such equipment. However, in lore there are also enchanted areas, or even larger objects that amplify spells or varieties of spells, that would be fun additions. To add to this concept though we can't forget we're talking about adding very unique spells to the game, such spells if powerful enough may themselves require new components, be it runes, or just rare elements consumed in their casting et cetera. While the requirement of material components for most TES spells has been excluded preferring a pure Magicka method, the use of such components with these new and exotic spells wouldn't break the TES tradition, as it would not only emphasize how special these spells are but also emphasize that they are being rediscovered. Perhaps upon advancing these types of magic further eventually you could stop with the material elements, just the way illusion eventually would lose the vocal and gestural elements when significantly advanced.
Finally I for type of spells, I would suggest permanent area effects of enchantment, illusions like de'javu in combination with an area of effect invisibility spell on a house or something would be great (Though useless in the TES world as no one ever comes looking for you or hunting you, however if they improve this system such an illusion, and old spells like the 'lock' spell might actually see practical use), or perhaps even spells that alter time, channeling as mentioned by many others, and even as an incredibly illusory sort of magic, which is really difficult to find, it would be fine it they added a small quest-line for some sort of dimensional rift spell or something. Many such adventures existed in the D&D world, but they are excessively complicated, and thus wouldn't be possible unless they had their own quest line attacked to them at the very least.