Also, completely deleting 9th level spells (and 8th) is a quick and tidy fix with little muss and fuss. However, there are plenty of not-overpowered 9th and 8th level spells and lots of overpowered spells that are available earlier. And this is an internet forum where we have the initiative and brain power to find the obscure fixes which work better than quick and tidy fixes.
We shouldn't be asking ourselves "what spells are overpowered?".
The question we should be asking ourselves is "Just how much power do we want the wizard to retain?"
I would like to prevent Joan Q. McWizard's Player from stating "I use my '
I win'
button spell". That's not fun for anyone. Either the Wizard is holding back their '
I win' button or everyone else feels like they aren't contributing when Joan Q. McWizard DOES use that spell.
So just getting rid of those broken options entirely would make me happier. I'm okay with Wizards having nice things. But nothing so nice that it amounts to little more than an "I win" button, rather than a tool which might lead to victory. Something which requires a little effort on the wizard's player's part instead of spells which are so effective in so many situations that the idea of tactical choice and forethought is annihilated. If the outcome of "Wizard wins the game" is guaranteed, we have a movie, not a game, gentlewomen and gentlemen. Interactivity and choice are the key words for games.
Another problem is "I waste it with my crossbow again and it dies" or "I full attack it again and it dies". Boring with a capital B. I have no idea how we're going to make combat complicated enough that "I full attack it again" isn't the best answer. Tome of Battle is a good idea for this. Deciding as a group to ban simple beatsticks from your games if "I full attack it again" sounds boring is a good idea.
There is no authority to ban spells from real life play on the internet. That's crazy talk. People are going to play at whatever power level they want to. Your group wants and all Wizard party playing rocket tag? Awesome sauce in my book. Your group wants an all Barbarian party playing hack and slash medieval combat simulator? Just as much awesome sauce in my book. The awesomeness of different power levels in DnD is different groups can find their optimum power level. And they will just house rule broken crap into the stone age or ban it when they don't want it.
The use in pointing out which spells are overpowered and how to limit spellcasters' power in the easiest way possible is that a lot of people want to play in a mixed party of stabby types and spellcasty types and would love an easy way to get those two balanced.
Forcing spellcasters to only pick spells from one school of magic works well (Example: I picked Evocation and can only ever cast Evocation spells).
They'd have to multiclass into other schools of magic to get access to other schools (Example: I'm an Evocation Wizard 10/Conjuration Wizard 10. That means I cast a 10th level Wizard that can only learn/prepare Evocation spells and as a 10th level Wizard that can only learn/prepare Conjuration spells).
And then balancing each school to about Evocation's level so everyone doesn't just pick Conjuration or Transmutation on their Wizard (as is detailed in this thread).
If everyone is cool with spellcasters losing their game breaking versatility, that's how you do it.
If everyone still wants some classes to be vastly more powerful than other classes, nothing needs to be changed.
Maybe if you combined this with ToB-style multiclassing between wizard variants? An evoker 10/conjurer 10 could cast 8th-level evocation and conjuration spells, while an evoker 5/conjurer 4/enchanter 1/illusionist 10 could cast 8th-level illusion spells, 7th-level evocation spells, 6th-level conjuration spells and 5th-level enchantment spells. Caster levels from all variants stack, and they share a single spell progression.
You might want to replace the wizard's bonus feats with minor school-specific benefits (a la Master Specialist) and reshuffle the contents of the schools a little.
PrCs which advance spellcasting have their rates halved, but caster level boosts remain the same (ie. an abjurer 10/incantatrix 10 would be casting as a 15th-level abjurer at CL20). Effectively they're another school in themselves.
Make the bonus from other school devotee levels 1/8 instead of 1/2 and I'd be happy. There should be some point in multiclassing, but it shouldn't be the obvious answer every single time. As you pointed out...
an evoker 5/conjurer 4/enchanter 1/illusionist 10 could cast 8th-level illusion spells, 7th-level evocation spells, 6th-level conjuration spells and 5th-level enchantment spells.
Would I take access to all those other schools instead of more 9th level spell slots in one school? Every single time. That's the definition of super-omega imbalanced. Just like one has to be an idiot to not take Natural Spell. There's one option so good that the choosing between them is not really a choice at all. (If your group likes having imbalanced options because they want to play rocket tag, that's totally cool. I'm writing for the people who want Wizards to be about as powerful as Tome of Battle.)
What does a 1/8 multiclassing bonus look like?
Evoker 17/Conjurer 1/Illusionist 1/Transmuter 1 has 9th level Evocation spells and 2nd level Conjuration/Illusion/Transmutation spells.
Alternatively...
Evoker 16/Conjurer 1/Illusionist 1/Transmuter 1/Abjurer 1 has 8th level E spells, 2nd level C/I/T/A spells.
At most we're getting +2 effective levels for 1 level dips (16/8=2, and 16 is the highest multiple of 8 under 20.).
That's 2nd level spells for 1 level dips. I think "Multiclass or Singleclass?" a tough choice. That's the definition of balanced - At least the class options are balanced against each other.
Now would I pick an Evoker 17/Conjurer 1/Illusionist 1/Transmuter 1 over a very well built Tome of Battle character? I don't know. I think they're competitive. Am I completely wrong?
If I am right, then with some more tweaking, I think we've come up with simple way to make Wizards Tier 3 for everyone who wants mixed parties of spellcasty types and stabby types.
