I think most players who've used these schools of magic at higher levels would agree they make everything very easy - too easy for most. They're not the only skills that can do this but they're what I'm going to focus on in this thread, since I've always been a wizard at heart in RPGs the failures of Skyrim's magic is one of the things that disappointed me most.

Conjuration

Problem #1. Enemy A.I. is just idiotic. There's no getting around this, they'll switch their attention to your summons far too easily. This is understandable for animals and so on, but more intelligent races and beings should have some concept of conjuration - enough to know that daedra that randomly appear in a colorful magical swirl will go away if their summoner dies. That doesn't mean they should completely ignore summons, just that summons shouldn't pretty much automatically keep everything away from you.

Problem #2. Spam-ability. In lore, in the many books you read in game at least, conjuration is made out to be a pretty serious thing. There's a major discrepancy between the picture painted in the game's literature, and what we actually experience in game play. We can almost instantly toss out daedra like we're casting something as simple as candlelight. The more powerful the summon, the more involved bringing it into our plane of existance should be. Summoning during combat should be possible, certainly, but not as simple as it is in Skyrim. Currently I can whip out two dremora lords with a one second cast. Kind of ruins the mystery, risk, and power involved in conjuration suggested in lore.

Illusion

Problem #1. Ease of use, all or nothing. Using an illusion spell is as simple as casting it on an NPC, and they're either affected or not based on their level for the complete duration. If they can be affected, they're under your complete control essentially. Since you can affect almost everything, this causes some obvious balance issues. Enemies having variable amounts of resistance would've been more appropriate, and less counter-intuitive as we cannot see enemy levels without using the console.

Problem #2. Cost. Cost of Illusion spells is consistently completely negated at high levels. In the 30 seconds of a pacify duration, you'll have regenerated all of the magicka spend back. In the 30 seconds of a frenzy's duration, it'll depend on your regen, but you'll still be getting most of your mana back while enemies are killing eachother. I think a fitting solution would be making every second cost a certain amount of magicka. Like a channeled spell, but you don't have to channel it actively your magicka just drains. The stronger the resistance of the enemy and the weaker your spell, the faster it drains. Keeping multiple enemies controlled would be more costly. This'd create a more appropriately tiered degree of power for Illusion spells, as well as prevent the potentially indefinite incapacitation of any enemy(other than Dragons).



Note 1: I'm leaving invisibility and muffle out of it, as I feel they're more related to stealth issues in general and sneak / sneak attacks have to be factored in.

Note 2: I left out bound weapons, but that's pretty simple: Bound weapons should scale with conjuration level to be appropriately competitive with mundane weapons, perhaps with methods of getting something equivalent in power to enchants on them via perks or additional spells. And obviously, we should've had more variety - a bound dagger, bound 2h sword, hammer, mace, etc. Considering they're just Daedric weapons with an ethereal looking effect(admittedly a nice one) on them how hard would that've been?