Just some random thoughts.

1.) Spells that have a limited duration should refresh their duration automatically as long as you have the spell still "ready". For example, Candlelight has a duration of 60 seconds. What I would have done is make that 60 seconds be constantly resetting itself as long as you actually have the Candlelight spell ready. It would drain magicka at a small but steady "per second" rate in the background, i.e. if the spell costs 30 magicka to cast then it consumes 30/60 = 0.5 magicka per second to "sustain" once established in order to keep resetting the timer. Once you put your spell away, then the duration timer actually starts going down, and the spell will end after its time is out. You can reset the timer automatically by re-equipping the spell. Candlelight, mage armor, summons, courage, pretty much anything with a duration would be much less annoying to use with this system because you wouldn't constantly be casting over and over again to keep your spells up.

2.) In Destruction, a single perk that adds percentile damage to ALL elemental attacks just like weapon skills, with the specialized "per element" perks adding an additional, larger bonus. Naturally.

3.) Wards should recharge while active. A ward might be able to absorb 40 damage, and replenish itself at a rate of 10 per second as long as its "battery" doesn't run out. A visual effect should show your ward's current strength. Further, the "idle cost" would be fairly low, offset by turning a percent of all incoming hostile magicka into a magicka cost. It might start out at 1/4, i.e. if you get hit by a spell that cost 60 magicka to cost it will cost you 15 to deflect it. The Restoration perk that improves wards would reduce this down to around 1/10th.

4.) Would have kept Resist/Weakness spell effects. Where the ward provides absolute defense at an ongoing magicka cost, Resists provide partial defense for a one-time magicka cost. And Weakness spells add some depth to tough fights.

5.) Would have kept Slow spell effect. Mages are squishy, and some way to tie up the ridiculously powerful melee enemies that want to one-hit you would be welcome.