I felt that the OP should be opinion free, so I'm making a separate post for mine.
1-First, I did miss Spellmaking in Skyrim. Ironically, the first spell I would change if I had the chance would be to have weaken the fireball spell by reducing/eliminating the radius of the blast. Of course that means maybe I just would have increased the strength of the firebolt.
2-Runes were nice. They should have been expanded and we should have been allowed to lay down more than one to begin with.
3-I liked both Oblivion's and Skyrim's Conjuration. I wish Oblivion's Summon Skeleton spells made it to Skyrim. An animation where the skeleton comes right out of the ground would been sweet.
4-I still miss Morrowind's Levitation. I also liked a Skyrim mod called Airstream Levitation. I think if such a spell was easier to aim on the move while making it available to NPCs, fights would have been a whole new kind of interesting ...if not hectic. Simple changes to level design could have prevented cheating in the few truly levitation sensitive locales.
5-The concept of Magicka-Free spells was terrible because the trade off seemed to be doing everything possible to then keep the damage output of those spells down. It would have been better to let spells grow in strength freely and make cost reduction come at the price of damage reduction at the same time. Not 1:1, but somewhere between 2:1 and 4:1 [cost reduction:damage reduction]
5b-But in all honesty, spells should never be free.
6-The balancing of Magic in Skyrim is terrible. Not that it is much better in Morrowind and Oblivion, but at least in those two games it broke in the player's favor.
6b-The balancing ...or lack thereof, for Magic in Skyrim was made more pronounced by its disparity to how melee and archery are balanced in the game. It is practically two different games.
7-Offensive Restoration spells [against undead] is an interesting idea that I hope continues.