If you could rewrite the magic system...

[Illumina Drathiran'ar]

While we're tinkering, here, I'd like to suggest we make Dispel Magic a universal spell. It seems like something all mages should be taught in mage school, doesn't it? "Now, if you turn your brother into a squirrel and your mother gets mad at her, here's how you bring him back."

In the context of Faerunian magic, Dispel Magic doesn't have to belong to a school when its purpose is to undo magic.

[SixOfSpades]

Illusionists jumping to the most powerful specialty if they get Divination as their Opposition school: Of course. Under the current setup, not being able to cast Divination spells is practically no disadvantage at all, which is why Conjurers are currently so powerful. There are no Divination spells after Level 6, and no Enchantment spells after Level 5, so those who have Divination or Enchantment as their Opposition schools have no drawbacks at all in the upper levels.

That's why I first came up with the idea of the Focus Mages: Like the Specialist Mages, but better. In brief, their advantages are that they can memorize up to 6 spells from every spell level (just like Specialists) and can learn every spell in the game. The disadvantages are that the first 2 spellslots of each level must contain a spell from their chosen School, and that they take a -75% penalty to their odds of scribing a spell from their Opposition school. Viola! Conjurers no longer kick ass because Divination is weak, but rather because Conjuration is strong (which it is). But they are also forced to play in character, casting from the Conjuration school at least 1/3rd of the time. This would, of course, require making new spells, so that every school has at least 1 spell present at each level.

Then again, it's also worth considering Divine Remix's approach to the situation, with each of the kits getting Major, Minor, or All access to each sphere of arcane magic. There might very well be an Anti-Magic sphere, with spells like Dispel Magic, Detect Invisibility, Breach, Pierce Shield, Stone to Flesh, etc. If each Mage kit recieved (at least) Minor access to the sphere of Anti-Magic, then they would all be able to cast Dispel Magic, as it is Level 3.

As for "Why would anybody plan an X or (X's opposition school) Y, when they could be anything else and be able to cast from both X and Y?"
Because they might not want to be denied spells from Z, P, L, Q, or N. Under "my" arrangement of the 8 schools, a Conjurer would be able to cast both Abjuration and Evocation, yes, but no Necromancy. Having no access to ADHW could be a serious bummer. Meanwhile, ADHW would be easily available to Abjurers and Invokers.

Sorcerers and Wild Mages are fine as they are, I think. Besides, you can't pin them down to being representative of any one school, that would defeat their entire purpose.

I think Sphere of Chaos is an Alteration spell because its effects include being hasted, randomly teleported, and being turned into a squirrel--and Haste, Teleport Field, and Polymorph Other are all Alteration spells.

[Shard]

I've not seen the Divine Remix way of doing things, I'll have a look.

I like the idea of a basic anti-magic sphere (or school) that all mages get access to.

On your idea of Focus Mages - I think having the first two spell slots as mandatory for your spell school is a great idea. You wouldn't necessarily have to make new spells (although that would be fun!) you could always move a few around. On top of the 75% scribing penalty, I would also add a chance of a wild surge (20% maybe?) when casting spells from your opposition school, just to add some extra danger

[Balintherlas]

Well what would be opposite Necromancy, its like death magic and there isnt a life magic school, i would something like that.