The Illusion spells for things like making a non-existent person would be nice, but not needed.
I didn't suggest this for a gameplay Need so much as for Lore. It just annoys me that the school of Illusion doesn't have any actual Illusions in it. In character, I guess you could assume that Fear is caused by making you look terrifying, calm less like an enemy, and rage makes them look like enemies to each other, but I'd just kind of like for there to be some sort of Illusion in Illusion to satisfy the fridge logic. I mean Alteration changes stuff, Conjuration Conjures things, Destruction Destroys people, Restoration Restores you... The only odd one out that doesn't do what it says on the tin is Illusion. Neither of the effects I mentioned would really change the game, other than making Illusion do what it implies.
I'd also add Elemental Defense to Destruction. Resist Fire, 60 seconds, level dependent percentage (or just use the Augment perks.) My original idea was to add the resistance to Cloak spells, so Flame Cloak would add Fire resistance, what with being surrounded in magical flame, and all. Make it a perk, Protective Cloak instead of Runemaster to add the element's resistance to Cloak Spells aught to do it, so it's not Automatic. That way, you run into a Flame Atronach, you could drop Flame Cloak on for protection, but it'd still be immune to the damage from it.
Some Absorbtion effects would be nice, too. Absorb Heat/Cold/Electricity like Spell Absorbtion, but elementally specific for Destruction. Absorb health/Stamina would probably be Restoration, I don't know if Absorb Magicka would be too OP, maybe if it was high drain, like Wards, but you could cast it offensively against Wizards.
More Alchemic spells (By which I mean Alteration, not Apothecary) such as Transmute Iron-Steel, and more stages to Gold. Instead of casting the same spell to go from Iron to Silver to Gold, make a Novice Iron-Silver, then Apprentice Iron-Steel, and Adept Silver-Gold?
Enchanting Spells, like Firebrand to add damage temporarilly to a weapon. (As previously suggested.) Move Soul Trap over here, and a Cloak type aura for Magic Resistance. Keep the Crafting as is, except for maybe more cast on strike weapon effects. (I can think of a lot of them!)
I forgot, 2, or more Teleportation spells. The one I was thinking of before would be Line of Sight, like a Summon, only instead of bringing in something nastly from Oblivion on that spot, transporting yourself to it to cross obsticals like rivers, valleys, and the like. Master so it's 2 handed, and takes a while to charge up, so it's not too OP. (Blinking in behind someone for a backstab a la Nightcrawler/Azazel.) Also, a Teleport Other to randomly send enemies away, with a risk of running into them again later. Maybe a swap spell to exchange your position for another NPCs'? Mark/Recal, that was such a wonderful way to get around Morrowind, but make it so you can't cast it in combat, so it's less of a get out of danger free card. And an autocast, like Avoid Death, only to return you home instead of reviving you on the spot.
I actually have ideas for enough spells, perks, and effects to at least double the current system...