I agree with you completely, the versatility of magic should be its major draw. Unfortunately a lot of the more useful spell effects have been made into shouts, e.g. Teleport --> Whirlwind Sprint, Disarm, Unrelenting Force, Storm Call, etc. What I would love to see is more variation in the Destruction tree beyond the simple 'elemental' magics. For example, since Dismay and Kyne's Peace are basically Shout versions of Illusion spells, I'd love to see spell versions of Marked for Death, e.g. a sort of '
Hex' spell that reduces enemy armor/ attack strength and does a weak 'magical poison' damage over time.
I like your Slow idea and the reduction in Impact's stagger chance down from 100% to maybe 50, signalling for players to rely on other methods of controlling the battlefield. I saw a really cool
Ice Wall mod video (http://www.youtube.com/watch?v=ymd8AkJndNU) which could be an effective way of managing enemies (obviously if it was only available as a higher level spell). Some leeching effect spells would be excellent as well - hit a weak Draugr with a 2HP / sec life drain and let it slowly die while powering you to face the Draugr Deathlord. Magicka draining projectile spells would be a very valuable tool against enemy mages as well and add an additional dimension to mage - mage fights beyond the simple "I can Ward longer than you can" games that are currently played. It's also laughable how shock spells are recommended as effective tools against mages - I am yet to see any mage that has >2x health compared to magicka.
In the Illusion school it would be nice to have an additional illusion on top of calm / fear / frenzy - something along the lines of
'command', which could let you effectively thrall an enemy for a short period of time and have it treat you like an ally (kinda like animal allegiance shout).
In terms of re-tweaking of current effects, reducing Impact's chance to something like 75% (not infallible, but not as low as Power Shot from the archery tree since you can shoot arrows quicker than destruction spells), and also putting a max 90% magicka cost reduction cap would both help revitalise mage gameplay imo.