Bring back spell failure chance.
Problem fixed.
No, it wouldn't fix the problem at all. The chance of spell failures didn't make anything any less unbalanced in Morrowind and it won't even if it returned in Skyrim. What it really served to do was make using magic much more annoying as you could potentially die due to failing a spell you had a 99% chance to cast successfully, simply because an invisible dice roll arbitrarily decided you should fail.
Magicka cost based on spell's power - isn't that the case already in Oblivion (with the addition of the cost decreasing as you increase skill)?
That's correct, and really, there's absolutely no reason to assume that it wouldn't be in Skyrim. It's pretty much common sense, honestly, stronger spells = greater magicka cost, this logic is hardly unique to the Elder Scrolls.
I'd agree that making certain spells take longer to cast than others would be a good idea, it just makes sense.
And I definately agree on taking damage being able to disrupt spell casting as well. Mages are supposed to try to avoid getting directly hit, after all, and the chance of being disrupted should depend on factors like your willpower, skill level, and fatigue at the time.
I'd say no to spell cool downs as that sort of thing is just annoying.
But it seems to me that in Oblivion, the most overpowered spells were invisibility and chameleon, and maybe some magical resistence and reflect type spells, so maybe that sort of spell could have a cap, similarly to armor rating in Oblivion (It may or may not work like that in Skyrim as well, we'll see.) after all, if you can't make yourself invincible to all non-magical attacks, why should you be able to make yourself impervious to magic? Back to chameleon and invisibility, I'd repeat the suggestions in previous threads of invisibility, as its name implies, making you impossible to SEE, but NPCs can still hear you, though they shouldn't be able to know your exact location, since they can't see you, but they could enter the "alerted" state, and start looking for you, the main exception is if there are enemies who don't rely on sight. To go together with this, NPCs should be able to cast detect life, or its equivalent, if they know the spell, and find you by using it. This way, invisibility and chameleon would no longer make sneak entirely obsolete, rather, they'd just be an alternative to it, or a spell that complements it, as they should be. And even if you're invisible, you wouldn't be able to just run right into NPCs without fear.
But honestly, I've never found magic as a whole overpowered in the Elderv Scrolls, in fact, I've always felt it is too weak. Certainly, magic CAN be overpowered, but only if you use specific spells or strategies, if you just play normally, there are a lot of spells that really aren't as useful as they should be. Damage spells do too little damage, making combat as a mage really annoying, summoning and fortifying spells don't last long enough, and when you actually get spells powerful enough to kill enemies in less than 50 hits, chances are they're magicka cost is so high you'll only be able to cast them twice before needing to fill yourself up with restore magicka potions. In fact, I often get the impression that the game is intentionally made to be biased against pure mages, the existence of the silence spell is a good example. The spell pretty much cripples mages for the duration of its effect, preventing them from casting spells. Yet warriors don't have a similar spell, there isn't a spell that stops you from attacking, sure, you might say paralysis does that, but it also woks on mages. Plus, with magical resistences, you can potentially become completely immune to damage of a specific type, yet there's no way to become completely immune to mundane damage, even the reflect damage effect in Oblivion only applied to melee weapons, yet in the end, only mages are the ones who get screwed over by being theoretically able to encounter enemies they can't harm at all. So I get the impression that the games are designed with the assumption that the player will play a battlemage or some other sort of character who can use magic but also has other ways of solving problems, as opposed to a pure mage. No wonder these kinds of characters can be so powerful.
Then, fix alchemy and infinite potion/spell spam as others have mentioned.
It seems to me like they already fixed that by limiting how many potions you could drink at once in Oblivion. I wouldn't say it's an ideal approach, but it does help to prevent players from making themselves impervious to any attacks that aren't one hit kills by drinking so many restore health potions that any damage heals before the enemy can attack again.
Morrowind spell failure was badly done. Spell failure isn't THAT bad when carefully tuned.
Do you have any examples of games with carefully tuned spell failure, then?