I don’t automatically favor any edition of AD&D over any other. Though, of course, I have my opinions, and one of the strongest is that 2nd edition utterly missed the mark regarding schools of magic and specialist wizards. 1st edition only had one “specialist”, the illusionist, who was as much a complete subclass to mage as ranger was to fighter, and druid was to cleric. It had its own unique spells, and a spell rank (level) cap of 7. Unfortunately, continuing that edition’s trend of taking optional rules of limited application and turning them into official rules, 2e decided all schools of magic were worthy of having mage kits devoted to them. It’s rare that you get the kind of failure they achieved by combining a poorly thought out idea with abysmal implementation such as this.
There are certain schools that no mage in his right mind would deliberately cut himself off from (at least in BG), namely Abjuration, Divination, and Illusion. Abjuration is a must because it contains virtually all of the spells that can keep a mage safe in combat; and Divination is the only way to counter the school of Illusion’s numerous spells that also, more indirectly, protect a mage. So right off the bat, we’ve got three of the eight schools in the Must-Have category, which effectively eliminates their Opposing schools as well from being viable specializations. There are further reasons for doing so, as will be discernable from the changes I make, but this is already enough to justify the overhaul. I’ll just put it out there that the idea and execution of opposition schools is one of the worst things ever introduced in a game franchise.
It’s more than just the nuts and bolts, though. Whatever their other purposes, games are meant to be fun, and how much fun would it be to play an Abjurer in BG2? Really? In PnP a Diviner could be extremely powerful; but in a computer game like BG, where classes play a neglible role outside of combat, it’d be deathly boring. Kits of other classes follow the tried-and-true formula of granting bonuses to the parent class while removing some of its main abilities. What do mage kits add? Nothing beyond an extra spell slot. What’s the cost? An entire school of magic, arbitrarily determined and assigned. The most ridiculous part is comparison between kits. The differences between an Inquisitor and Cavalier are numerous, and make each distinct. The difference between an Illusionist and a Conjurer? Nothing except the particular school of magic each can’t cast.
The fundamental question has two parts: can you make a mage kit unique without it being over- or underpowered? And can you do so based on spell schools? The first question has some points of subjectivity, but only four current schools have any shot at being useful as a kit: Alteration, Conjuration, Evocation, and Necromancy. You could make a case for Abjuration as sort an anti-mage party buffer, but that’s shaky, and it’s the best candidate of the remaining four. But of those four, only Enchantment doesn’t contain at least one critical spell no mage would do without, so while Abjuration, Divination, and Illusion are not good candidates for specialization, neither are they acceptable as opposition schools.