I would love to see more variance in AoEs, like a cone for ice, or a line for lightning.
And fire spells actually behaving differently than ice spells, and not being just another kind of damage that you have to choose based on the immunities of the opponent. Same for sound, acid, lightning ... wait, do we even have acid and sound ? I dont remember them right now in TES.
Or Chameleon any useful if its not 100%, or not being hopelessly overpowered once you reach 100%.
Magic Resistance 100% not translating to "no magic can touch you, EVER". Magic Resistance should only work on pure magic effects that effect you directly, such as Frenzy. A fireball is fire created from magic, but its not a form of magic - it is the real thing.
Magic Resistance not lowerable below 0%. How is that supposed to work, anyway ? Elemental Resistance not lowerable below 0%, except if you have a corresponding creature type.
Resistances not stacking linearily. If you have two items of 50% resistance, your total resistance should be 75%, not 100%. You only get to 100% if you have an item with 100% resistance.
Magic Reflection and Magic Absorption not translating into total immunity from any magic, even area spells. They should only reflect certain amounts of magic, then collapse. As item enchantments, they shouldnt be possible in the first place.
Damage Reflection should let the full damage through, should be capped at 50%, and should have some other drawbacks.
Defenses against Arrows. Seriously, you can get completely immune against magic and melee damage in TES, but there is no defense at all against Arrows ?
Buffs that you cast once, block a fixed amount of mana, and then continously operate with needing refreshment.
Recast timers on spells, and a limit on total spells you can have. Of course you can reconfigure your spelllist outside combat any time by storing and loading spells from spellbooks, but you shouldnt end up with hundreds of spells in your spelllist and having to scroll through them all the time to find the spell you want to use (other than the ~8 you can have in a quickslots).