I would like to re-introduce the Morrowind spellcrafting and have the attributes max/min (target, touch, self, channel; fire, ice, light; aoe, duration, magnitude, cast time; etc.) level based on use.
Instead of reading books to learn spells, attributes would be unlocked by books, trainers, or reaching certain levels (self taught), then the spells can be custom made.maybe have "recipes" for good spell settings laying around like they already do for potions.
It's not necessarily a spell, but I wish that you could use the phantom decoy Shout the Greybeards use to test your Thu'um. It would be quiet, like Aura Whisper, and function like an attention decoy. Your enemies would attack it, but it would not attack back, allowing you to slip by, etc. Perfect for some builds, like David Okine's Wraith Ranger.
1) A spell to summon a Daedric prince to fight by your side once you become his/her champion.
2) Augment your gauntlets with elemental magic to deliver elemental punches.
3) A spell to send Nazeem directly to the Cloud District. Everytime I speak to any NPC on Whiterun, he simply stands between me and the NPC and speaks his signature quote.